Add more notes regarding the uniform setters

Having overloads for GLuint that call glUniformNi was a mistake to begin with,
and it has only been made worse when later OpenGL versions introduced unsigned
int support and glUniformNui. Note this in the docs.

Task-number: QTBUG-37012
Change-Id: Icc867221d0fbced8c4ff769deee66effa022f1b5
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>