Missing documentation for FilePath

[thijs]

The  documentation for FilePath has some "No summary" and "Undocumented" methods listed, the attached patch fills in those blanks.

[tom.prince]

* {{{_PathHelper}} doesn't actually exist (it is referenced in one the doc strings

* There are quite a few places that change L{} to C{} for no apparent reason. Or introduce C{} where L{} would work.

*  twistedchecker reports a number of docstring errors after this patch (although less than before).

Feel free to commit after fixing.

Also,

* There are a number of places (basename and splitext for example, where the description of the function and the return value are essentially the same. It would be nice to simplify one of them (probably the return value).

[exarkun]

(In [36624]) Branching to 'filepath-docs-6046'

[exarkun]

(In [36625]) Apply filepath-doc-6046.patch

refs #6046

[exarkun]

(In [36626]) adjust a bunch of C{} to L{}; fix some other minor issues

refs #6046

[exarkun]

(In [36628]) some more doc fixes, mostly pointed out by twistedchecker

refs #6046

[exarkun]

 Build results

[exarkun]

(In [36631]) Merge filepath-docs-6046

Author: thijs, exarkun Reviewer: tom.prince Fixes: #6046

Various improvements to the API documentation content and markup of twisted.python.filepath.