doc: convert git-restore to new style format

doc: convert git-restore to new style format

- Switch the synopsis to a 'synopsis' block which will
  automatically format placeholders in italics and keywords in
  monospace

- Use _<placeholder>_ instead of <placeholder> in the description

- Use backticks for keywords and more complex option
descriptions. The new rendering engine will apply synopsis rules to
these spans.

While at it, also convert an option description to imperative mood.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>