framebuffer: Remove _EXP symbol mangling and add gtk-doc
framebuffer: Remove _EXP symbol mangling and add gtk-doc

We are in the process of removing all _EXP suffix mangling for
experimental APIs (Ref: c6528c4b6c3c34) and adding missing gtk-doc
comments so that we can instead rely on the "Stability: unstable"
markers in the gtk-doc comments. This patch tackles the cogl framebuffer
symbols.

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