libgimp: have both annotations for GObject Introspection and gtk-docs.

GTK-docs needs the struct member documentations in the main struct
comment. So let's get this back, while keeping the proper GI
introspection for each method this time.
As discussed with Mitch.
3 jobs for master in 16 minutes and 5 seconds (queued for 1 second)
Status Job ID Name Coverage
  Babl
passed #427346
babl-git

00:02:18

 
  Gegl
passed #427347
gegl-git

00:04:43

 
  Gimp
passed #427348
build-git-autotools

00:09:03