Replace "Return" with "Returns" in API documentations

The guidelines are explicit about how we should start sentences with
"Returns" rather than "Return." So these instances should follow that way.

See also the enhancements at rails#50595.

Co-authored-by: Jonathan Hefner <[email protected]>

Author: sato11

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

@@ -475,8 +475,7 @@ def num_waiting_in_queue # :nodoc:
         @available.num_waiting
       end
 
-      # Return connection pool's usage statistic
-      # Example:
+      # Returns the connection pool's usage statistic.
       #
       #    ActiveRecord::Base.connection_pool.stat # => { size: 15, connections: 1, busy: 1, dead: 0, idle: 0, waiting: 0, checkout_timeout: 5 }
       def stat

activerecord/lib/active_record/connection_handling.rb

@@ -256,7 +256,7 @@ def connection
 
     attr_writer :connection_specification_name
 
-    # Return the connection specification name from the current class or its parent.
+    # Returns the connection specification name from the current class or its parent.
     def connection_specification_name
       if @connection_specification_name.nil?
         return self == Base ? Base.name : superclass.connection_specification_name