doc: apply new documentation guidelines to git commit

- 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.

Signed-off-by: Jean-Noël Avila <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jnavila