docs: improve docs and guides

Fix examples and claims that no longer match the code (callback arities
in the adapter guide, return values, event types, option lists) and drop
the marketing tone across the module docs, guides, and README.

Author: cabol

CHANGELOG.md

@@ -55,7 +55,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
   `use Nebulex.Cache` now validates at compile time that the adapter lists
   `Nebulex.Adapter`, `Nebulex.Adapter.KV`, and `Nebulex.Adapter.CompositeKV`
   as behaviours. Add `use Nebulex.Adapter.CompositeKV` to an adapter for the
-  default implementation.
+  default implementation. The compile-time error says how to fix it: add
+  the `use` to the adapter, or upgrade it to a version built for v3.1.
   [#254](https://github.com/elixir-nebulex/nebulex/issues/254).
 - [Nebulex.Cache] `get_and_update/3` no longer treats a `nil` new value as
   "do not write". A function returning `{current, nil}` now stores `nil`

README.md

@@ -15,19 +15,15 @@
 
 ## 🚀 About
 
-Nebulex provides support for transparently adding caching to existing
-Elixir applications. Like [Ecto][ecto], the caching abstraction allows
-consistent use of various caching solutions with minimal impact on your
-code.
-
-Nebulex's cache abstraction shields developers from directly interacting with
-underlying caching implementations, such as [Redis][redis],
-[Memcached][memcached], or other Elixir cache implementations like
-[Cachex][cachex]. It also provides out-of-the-box features including
-[declarative decorator-based caching][nbx_caching],
+Nebulex adds caching to Elixir applications. Like [Ecto][ecto], it is an
+abstraction: you use different caching backends through the same API, with
+little impact on your code.
+
+You write against the Nebulex API instead of talking to [Redis][redis],
+[Memcached][memcached], or an Elixir cache like [Cachex][cachex] directly.
+Nebulex also comes with [declarative decorator-based caching][nbx_caching],
 [cache usage patterns][cache_patterns], and
-[distributed cache topologies][cache_topologies],
-among others.
+[distributed cache topologies][cache_topologies].
 
 [ecto]: https://github.com/elixir-ecto/ecto
 [cachex]: https://github.com/whitfin/cachex
@@ -75,19 +71,17 @@ def deps do
 end
 ```
 
-To provide more flexibility and load only the needed dependencies, Nebulex makes
-all dependencies optional, including the adapters. For example:
+All dependencies are optional, including the adapters, so you only load what
+you need:
 
-  * **For enabling [declarative decorator-based caching][nbx_caching]**:
-    Add `:decorator` to the dependency list (recommended).
+  * For [declarative decorator-based caching][nbx_caching], add `:decorator`
+    (recommended).
 
-  * **For enabling Telemetry events**: Add `:telemetry` to the dependency list
-    (recommended). See the [Info API guide][info_api] for monitoring cache stats
-    and metrics.
+  * For Telemetry events, add `:telemetry` (recommended). See the
+    [Info API guide][info_api] for cache stats and metrics.
 
-Then run `mix deps.get` in your shell to fetch the dependencies. If you want to
-use another cache adapter, just choose the appropriate dependency from the table
-above.
+Then run `mix deps.get`. To use another cache adapter, pick its dependency
+from the [adapters guide][nbx_adapters].
 
 Finally, in your cache definition, you'll need to specify the `adapter:`
 corresponding to the chosen dependency. For the local cache, it would be:
@@ -120,18 +114,14 @@ iex> MyApp.Cache.fetch("foo")
 {:ok, "bar"}
 ```
 
-For more detailed information, see the
-[getting started guide][getting_started-rc1] and
-[online documentation][hex-docs].
-
-[getting_started-rc1]: https://hexdocs.pm/nebulex/getting-started.html
-[hex-docs]: https://hexdocs.pm/nebulex/Nebulex.html
+For more details, see the [getting started guide][getting_started] and the
+[online documentation][docs].
 
 ---
 
 ## ⚡ Quick Start Example with Caching Decorators
 
-This example demonstrates how to use Nebulex with Ecto and declarative caching:
+This example uses Nebulex with Ecto and declarative caching:
 
 ```elixir
 # In config/config.exs
@@ -244,14 +234,14 @@ To run only the tests:
 $ mix test
 ```
 
-Additionally, to run all Nebulex checks:
+To run all the checks CI runs:
 
 ```bash
 $ mix precommit
 ```
 
-The `mix precommit` command will run the tests, coverage, credo, dialyzer,
-and more. This is the recommended way to test Nebulex.
+The `mix precommit` alias runs the tests, coverage, Credo, Dialyzer, and the
+other checks. Run it before opening a pull request.
 
 ---
 
@@ -267,15 +257,14 @@ To run a benchmark test:
 $ mix bench
 ```
 
-> The benchmark uses the `Nebulex.Adapters.Nil` adapter; it is more focused on
-> measuring the Nebulex abstraction layer performance rather than a specific
-> adapter.
+> The benchmark uses the `Nebulex.Adapters.Nil` adapter, so it measures the
+> Nebulex abstraction layer, not a specific adapter.
 
 ---
 
 ## 🤝 Contributing
 
-Contributions to Nebulex are very welcome and appreciated!
+Contributions are welcome.
 
 Use the [issue tracker](https://github.com/elixir-nebulex/nebulex/issues)
 for bug reports or feature requests. Open a