Improve while_preventing_writes documentation

Adds a note to clarify that `while_preventing_writes` is not meant to be
a replacement for a readonly replica user.

Closes #39132 and #39133

Co-authored-by: Eike Send <eike.send@gmail.com>

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -1035,6 +1035,12 @@ def prevent_writes=(prevent_writes) # :nodoc:
       # In some cases you may want to prevent writes to the database
       # even if you are on a database that can write. `while_preventing_writes`
       # will prevent writes to the database for the duration of the block.
+      #
+      # This method does not provide the same protection as a readonly
+      # user and is meant to be a safeguard against accidental writes.
+      #
+      # See `READ_QUERY` for the queries that are blocked by this
+      # method.
       def while_preventing_writes(enabled = true)
         original, self.prevent_writes = self.prevent_writes, enabled
         yield