doc: git-checkout: clarify restoring files section

doc: git-checkout: clarify restoring files section

From user feedback on this section: 3 users don't know what "tree-ish"
means and 3 users don't know what "pathspec" means. One user also says
that the section is very confusing and that they don't understand what
the "index" is.

From conversations on Mastodon, several users said that their impression
is that "the index" means the same thing as "HEAD". It would be good to
give those users (and other users who do not know what "index" means) a
hint as to its meaning.

Make this section more accessible to users who don't know what the terms
"pathspec", "tree-ish", and "index" mean by using more familiar language,
adding examples, and using simpler sentence structures.

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