doc: deprecate cogl_texture_get_rowstride()
doc: deprecate cogl_texture_get_rowstride()

This documents that cogl_texture_get_rowstride() is deprecated (or
rather it was a mistake that the api was ever published) and also
clarifies the rowstride argument documentation for
cogl_texture_get_data() to explain how it's automatically calculated
when 0 is passed to help avoid misleading people into thinking that
cogl_texture_get_rowstride() is an appropriate way to get a valid
rowstride for that.

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

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