Document what the stage numbers in the :$n:path syntax mean.

Document what the stage numbers in the :$n:path syntax mean.

The git-rev-parse manpage talks about the :$n:path notation (buried deep in
a list of other syntax) but it just says $n is a "stage number" -- someone
who is not familiar with the internals of git's merge implementation is
never going to be able to figure out that "1", "2", and "3" means.

Signed-off-by: Junio C Hamano <redacted>