doc: am: fill out hook discussion

doc: am: fill out hook discussion

Document `--verify` and rephrase the `--[no-]verify` section to lead
with the default, in imperative mood.[1]

Historically it makes sense that only the negated forms are documented;
they are all run by default and thus you only need to use hook options
if you want to turn some of them off. But, beyond just desiring uniform
documentation,[2] it’s very much possible to have, say, a Git alias with
`--no-verify` that you might sometimes want to turn back on with
the *positive* form.

Also mention the options in the “Hooks” section and mention that
`post-applypatch` cannot be skipped.

† 1: See e.g. acffc5e9 (doc: convert git-remote to synopsis style,
     2025-12-20)
† 2: https://lore.kernel.org/git/xmqqcyct1mtq.fsf@gitster.g/

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>