Doc: Always treat \brief as full sentence

Make sure all \brief descriptions start with an upper-case letter and end with a .

Also start descriptions of \class with the name of the class or struct.

Change-Id: Ifd2656201f9c1dff092085508a5423ce516e2d3f
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>