various minor fixes, doc-string fixes, menu improvements, and a new command

Here's a set of changes I've worked on recently to improve Magit for my own use.

I think most of the doc-string fixes should be quite straight forward improvements to language and content, etc.

I've improved the main menu, adding and re-arranging items so that I can use it as a better reminder to myself of how to do things.

There are some new bindings for the "alternate" of rigid/non-rigid bindings so that all commands are still available.

There's also a new command to help my finger memory continue to get something akin to the same behaviour for "=" that's found in the likes of PCL-CVS.

Pick-and-pull as desired!  Thanks in advance!