Doc: update description of qmake SUBDIRS variable

You can specify a relative path to a .pro file in any directory.

Fixed a capitalization issue and a grammar issue and changed the
visible text in a link target.

Task-number: QTBUG-14139
Change-Id: Ib476bd400c46645709950c582492edcaaba9d7ed
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>