man.1: A big update of the notmuch manual page.

I set out merely to add documentation for the recently-added options
for "notmuch search" (--first, --max-threads, and --sort), but ended
up revamping a lot. A significant change is a new SEARCH SYNTAX
section separate from "notmuch search" that is referred to in the
documentation of search, show, reply, and tag.

Also many sections were updated to reflect recent changes, (such as
the dropping of the NOTMUCH_BASE environment variable, the addition of
the .notmuch-config file, etc.)