git-rm doc: Describe how to sync index & work tree

git-rm doc: Describe how to sync index & work tree

Newcomers to git that want to remove from the index only the
files that have disappeared from the working tree will probably
look for a way to do that in the documentation for 'git rm'.

Therefore, describe how that can be done (even though it involves
other commands than 'git rm'). Based on a suggestion by Junio,
but re-arranged and rewritten to better fit into the style of
command reference.

While at it, change a single occurrence of "work tree" to "working
tree" for consistency.

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>