I use "enhancement" for non-trivial documentation patches.

Also, quoting from https://docs.python.org/devguide/triaging.html#type

"Also used for improvements in the documentation and test suite and for other refactorings."

In this case, your patch fixes a documentation issue and improving current documentation by documenting {Lock,RLock}.acquire() and release() methods properly (documentation improvements can go into bugfix branches).