[CHORE] Say each invariant once in the comments

The comments carried the reasoning that found the bug as well as the rule the
code follows. The rule is what a reader needs; the rest belongs in the pull
request. Each block now states its constraint and stops, and the two member
comments in the installed headers follow the one line trailing form the file
already uses next to them.

No code changes.

Signed-off-by: thc1006 <84045975+thc1006@users.noreply.github.com>

Author: thc1006

ext/src/http/client/curl/http_client_curl.cc

@@ -817,10 +817,8 @@ bool HttpClient::doRemoveSessions()
         continue;
       }
 
-      // Take it out of the multi handle first. libcurl does not allow an easy handle to be
-      // cleaned up while a multi handle still owns it, and the header list has to stay alive
-      // until the handle is no longer used for a transfer. A handle that was never added
-      // reports CURLM_OK here, so this does not need to know which is which.
+      // Remove before cleanup: libcurl forbids freeing an easy handle, or the header list it
+      // points at, while the multi handle still owns it. A handle never added reports CURLM_OK.
       const CURLMcode rc = curl_multi_remove_handle(multi_handle_, resource.easy_handle);
       if (CURLM_OK != rc)
       {

ext/src/http/client/curl/http_operation_curl.cc

@@ -542,10 +542,8 @@ void HttpOperation::Cleanup()
   // Only cleanup async once even in recursive calls
   if (async_data_)
   {
-    // Move the easy handle to the owner untouched. It may still be attached to the multi handle
-    // and running, and the thread inside curl_multi_perform owns it until the IO thread takes it
-    // out, so writing to it here would be a write to a live transfer. It is pointless as well:
-    // the next thing that happens to the handle is curl_easy_cleanup.
+    // Hand the easy handle over untouched. It may still be attached to the multi handle, and the
+    // thread inside curl_multi_perform owns it until the IO thread removes it.
     Session *session = async_data_->session.exchange(nullptr, std::memory_order_acq_rel);
     if (session != nullptr)
     {
@@ -1525,9 +1523,8 @@ void HttpOperation::PerformCurlMessage(CURLcode code)
 {
   if (is_cleaned_.load(std::memory_order_acquire))
   {
-    // Already torn down, and the handle is queued for removal. The multi handle can still hold a
-    // message buffered from before that, and acting on it would dispatch a second round of
-    // terminal events and can put a handle that is waiting to be freed back on the retry queue.
+    // Already torn down and queued for removal. A message buffered before that would dispatch a
+    // second round of terminal events and requeue a handle waiting to be freed.
     return;
   }