docs: make cogl.h matrix stack documentation consistent
docs: make cogl.h matrix stack documentation consistent

This updates the cogl.h matrix stack documentation consistent with the
corresponding documentation in cogl-framebuffer.h for the framebuffer
matrix stack methods.

Reviewed-by: Neil Roberts <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

collects/cogl,djdeath/cogl,djdeath/cogl-android,gcampax/cogl,gcampax/cogl,collects/cogl,Distrotech/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl,Distrotech/cogl,Distrotech/cogl,gcampax/cogl,Distrotech/cogl,djdeath/cogl,djdeath/cogl-android