docs: improve project-objects & worktrees layout info

Make it clear that the paths have a .git suffix, and clarify the
reason for not using remote servers in the layout.

Change-Id: I62c6977ee6f4e1e9882d45727eb239cf5489d2b6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256033
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>