The reference to running "make coverage" in a subdirectory was a
bit obscure, so clarify what happens when you do that.  Do a little
desultory copy-editing, too.

Per a question from Peter Smith.

Discussion: https://postgr.es/m/CAHut+Pu0r3AjRSyu5E0v2-zRj8r24OSrkWs3fEBxOuaw1i8DKA@mail.gmail.com