doc: git-checkout: clarify ARGUMENT DISAMBIGUATION

doc: git-checkout: clarify ARGUMENT DISAMBIGUATION

There's no need to use the terms "pathspec" or "tree-ish" in the
ARGUMENT DISAMBIGUATION section, which are terms that (from user
feedback on this page) many users do not understand.

"tree-ish" is actually not accurate here: `git checkout` in this case
takes a commit-ish, not a tree-ish. So we can say "branch or commit"
instead of "tree-ish" which is both more accurate and uses more familiar
terms.

And now that the intro to the man pages mentions that `git checkout` has
"two main modes", it makes sense to refer to this disambiguation section
to understand how Git decides which one to use when there's an overlap
in syntax.

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