Documentation improvements for the matching module
The docstring for the whoosh.matching module does not end up in the HTML documentation because it is written after the imports in the source file, which means it was no longer recognised as a docstring since those always have to come first.
Please find attached a patch that fixes this. While at it, I also took the opportunity to
- fix a few grammar issues;
- add some more hyperlinks to referenced classes and methods;
- perform some typographical and stylistic nitpicks; and
- remove trailing whitespace.