Tweak documentation for the CoglTexture interface

The following changes are made to the documentation for CoglTexture:

• The description of the default value for the components property is
  changed to say that it is always RGBA for textures created by the
  ‘_with_size’ textures. Previously it said that the default is based
  on the pixel format used the first time data is set on the texture,
  but this is only true if the data is set using a constructor.

• Added documentation for the CoglTextureComponents enum.

• Changed it to say that it _specifies_ what components are required
  for sampling rather than determinging [sic] them.

• Added ‘Since: 1.18’ to
  cogl_texture_{set,get}_{components,premultiplied}

• Changed the since tag for CoglTextureError from 2.0 to 1.8.

• Added documentation for COGL_TEXTURE_ERROR_{FORMAT,TYPE}.

• Added the following to the cogl2-sections.txt file:
  COGL_TEXTURE_ERROR
  CoglTextureError
  cogl_texture_allocate
  cogl_texture_set_components
  cogl_texture_get_components
  cogl_texture_set_premultiplied
  cogl_texture_get_premultiplied

Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 2f12c4329c519fa14b927b2dcd708dddcc903c32)

doc/reference/cogl-2.0-experimental/cogl-2.0-experimental-sections.txt

@@ -400,6 +400,11 @@ CoglTexture
 cogl_is_texture
 
 <SUBSECTION>
+COGL_TEXTURE_ERROR
+CoglTextureError
+
+<SUBSECTION>
+cogl_texture_allocate
 cogl_texture_get_width
 cogl_texture_get_height
 cogl_texture_is_sliced
@@ -407,6 +412,11 @@ cogl_texture_get_data
 cogl_texture_set_data
 cogl_texture_set_region
 CoglTextureType
+CoglTextureComponents
+cogl_texture_set_components
+cogl_texture_get_components
+cogl_texture_set_premultiplied
+cogl_texture_get_premultiplied
 
 <SUBSECTION Private>
 COGL_TEXTURE_MAX_WASTE