doc: git-checkout: split up restoring files section

doc: git-checkout: split up restoring files section

From user feedback: one user mentioned that "When the <tree-ish> (most
often a commit) is not given" is confusing since it starts with a
negative.

Restructuring so that `git checkout main file.txt` and
`git checkout file.txt` are separate items will help us simplify the
sentence structure a lot.

As a bonus, it appears that `-f` actually only applies to one of those
forms, so we can include fewer options, and now the structure of the
DESCRIPTION matches the SYNOPSIS.

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>