git-arc: Try to improve documentation

When I originally wrote git-arc, the usage string was the documentation.
Later, a man page was added, largely copied from the usage string. Over
time they have diverged slightly.

It is silly to have two copies of the same information, so let's make
the man page the source of truth.

While here, document some options for git-arc-patch and rewrite a few
sentences here and there in a way that I hope is clearer.

Reviewed by: ziaee, ngie, adrian
Differential Revision: https://reviews.freebsd.org/D54405