buffer: Improve the CoglBuffer API description
buffer: Improve the CoglBuffer API description

This improves the gtk-doc overview for the CoglBuffer interface.

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

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