Documentation: do not treat reset --keep as a special case

Documentation: do not treat reset --keep as a special case

The current treatment of "git reset --keep" emphasizes how it
differs from --hard (treatment of local changes) and how it breaks
down into plumbing (git read-tree -m -u HEAD <commit> followed by git
update-ref HEAD <commit>).  This can discourage people from using
it, since it might seem to be a complex or niche option.

Better to emphasize what the --keep flag is intended for --- moving
the index and worktree from one commit to another, like "git checkout"
would --- so the reader can make a more informed decision about the
appropriate situations in which to use it.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>