Documentation edits

Edit the documentation for minor grammar, idiomatic usage, and sentence construction issues.  Overall the documentation is great;  this is just cleaning up some places that sound a little odd.

There are some notes left in the documentation delimited by [bwb: ...] where I made a larger-than-usual change or I thought needed attention from someone more familiar with the codebase than I am.


There are two spaces between "mechanism." and  "Instead".