git-{var,write-tree} docs: update mark-up of synopsis option descriptions

git-{var,write-tree} docs: update mark-up of synopsis option descriptions

To unify mark-up used in our documentation to a newer convention,
started by 22293895 (doc: apply synopsis simplification on git-clone
and git-init, 2024-09-24), update the documentation for 'git var' and
'git write-tree' to

 * use [synopsis], not [verse] in the SYNOPSIS section
 * enclose `--option=<value>` in backquotes

Signed-off-by: Junio C Hamano <redacted>
Helped-by: Jean-Noël Avila <redacted>
Signed-off-by: Junio C Hamano <redacted>