scmutil: improve documentation of revset APIs I can never remember the differences between the various revset APIs. I can never remember that scmutil.revrange() is the one I want to use from user-facing commands. Add some documentation to clarify this. While we're here, the argument name for revrange() is changed to "specs" because that's what it actually is.

mdiff: remove use of __slots__ The use of __slots__ was added way back in 2006 in 4ec58b157265. __slots__ isn't necessary for this class.