vcl: Document GetTextBoundRect() / GetTextWidth() / GetTextHeight().

GetTextBoundRect() fits different situations than the GetTextWidth() /
GetTextHeight() combination, so make it clear in the documentation.

These are complex enough that people shouldn't read the code just to
understand the difference, and misunderstanding of purpose of each leads
to visually not nice results.

Change-Id: Ida71477cdffbd8290328551bd6ddb4805b96c415
Improve computation of the text area to make it nicer.

Change-Id: I24fb1caedc55dcc297fb013acb63b3c660fcf0a2