Doc: Update description of qmake LIBS variable

Rewrite the note about specifying libraries on Windows
to make it clearer and change the library name to match the
one in the snippet.

Task-number: QTBUG-30229

Change-Id: I55c5579f5ead2fadc5d75baa2172b03f20c9f689
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>