Added missing docs for +supports_primary_key?+ on some adapters

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -219,6 +219,7 @@ def supports_migrations? #:nodoc:
         true
       end
 
+      # Does MySQL support finding primary key on non-Active Record tables?
       def supports_primary_key? #:nodoc:
         true
       end

activerecord/lib/active_record/connection_adapters/sqlite_adapter.rb

@@ -77,6 +77,7 @@ def supports_migrations? #:nodoc:
         true
       end
 
+      # Does SQLite support finding primary key on non-Active Record tables?
       def supports_primary_key? #:nodoc:
         true
       end