Set rules for PSBT serializer backcompat

The serializers are not thoroughly documented, but we want to keep their
backward compatibility. However, we reserve the right to update their
semantics to more fitting types once we implement them (e.g. script tree
leaf class)

Author: bbrtj

Changes

@@ -38,8 +38,8 @@ Revision history for Perl extension Bitcoin::Crypto.
 		- serialization of transactions with zero inputs and zero outputs is allowed (as required by some test cases)
 
 	[Breaking changes]
-	- PSBT_IN_FINAL_SCRIPTWITNESS is now serialized as an array reference of bytestrings
-	- PSBT_IN_TAP_LEAF_SCRIPT value is now serialized as an instance of Bitcoin::Crypto::Script::Tree::Leaf
+	- PSBT_IN_FINAL_SCRIPTWITNESS value serializer is upgraded to use an array of bytestrings
+	- PSBT_IN_TAP_LEAF_SCRIPT value serializer is upgraded to use an instance of Bitcoin::Crypto::Script::Tree::Leaf
 
 4.005 Thu Apr 23, 2026
 	[Public interface changes]

lib/Bitcoin/Crypto/PSBT/Field.pm

@@ -250,6 +250,13 @@ holds the descriptions of what is the type after deserialization (and before
 serialization). Reading the source of L<Bitcoin::Crypto::PSBT::FieldType> may
 be required if it isn't clear how they are implemented for a specific field.
 
+Even though specific serializers aren't documented, they will be kept
+backward-compatible with the same rules as any other part of Bitcoin::Crypto
+(see L<Bitcoin::Crypto::Manual/Backward compatibility>), with an exception of
+upgrading their behavior to more fitting types when they are implemented. If
+you are unsure whether the given serializer will be modified in the future, you
+can use C<raw_*> attributes and serialize/deserialize manually.
+
 Reading the value through L</raw_value> will return a bytestring, but reading
 through C<value> will use the deserializer. Calling C<set_value> will use the
 serializer to update L</raw_value>. The field only holds raw data and uses