Fix formatting of shell functions

Use Nm and Qq to format shell functions and their arguments instead of Fn.
This is a follow up to c15f917361dff317d77c66c7c35c332e13ffa20d.
Improve atf-sh-api(3)'s and atf-check(1) docs

Clarify how atf_check (a function in atf-sh) relates to atf-check(1) (the
standalone, private utility).

Fixes #12.