ENH: Add StringMethod.find and rfind
Derived from #9111.

Thanks, fixed.
 OK, looks good to me.
 @sinhrks I added some documentation comments. Further:
- I think you may group all the StringMethods enhancement in a section under 'new features' (as it starts to become a large list)
- Maybe we should start adding `versionadded` directives (see [here](http://sphinx-doc.org/markup/para.html#directive-versionadded)) to newly added functions? We don't do this consistently right now, but I think we should do that more.
 and another:
- maybe add a 'Notes' section saying it is equivalent to the standard library's `str.find` (I even think you can link to that using `:func:`str.find``
 Modified once.
- About release note, let me do once finish #9111 (added as AI).
- Adding `Notes` is duplicated with the "Method Summary" in `text.rst`. How about adding `Notes` in each docstring, and change "Method Summary" section to a simple link to "String handling" section in `api.rst`. 
 And on the note, I don't think that is a problem that is would be duplicated (that is the point of having both docstrings and narrative docs, there will always be some duplication). 

If the table in text.rst can be replaced by an autosummary table is separate issue I think. Positive side is that it would add the explanation manually, but at the moment I like more how the table looks in text.rst (better overview) than how it is on the api pages. 
 It is maybe not necessary to add it in a Note, just adding 'equivalent to standard library's str.find' in the explanation in the beginning of the docstring is also good.
 Maybe my word choice is inappropriate. What I'm caring is uniformity of styles and descriptions. What I expect is all the docstring should have similar format, and sufficient information in a single place.

In this respect, current doc should be revised in separate PR to cover:
- All the docstring should provide "better overview" and this should be used both in "text.rst" and "api.rst".
- Add descriptions for standard `string` methods (not only `find` and `rfind`). 

This is one thing, and added a description for this PR.
 thanks @sinhrks 
