gsk: API docs fixes

gsk: API docs fixes

gsk/gskenums.h:181: Error: Gsk: multiple "@GSK_TRANSFORM_CATEGORY_2D" parameters for identifier "GskTransformCategory":
 * @GSK_TRANSFORM_CATEGORY_2D: The matrix is a 2D matrix. This is equivalent
    ^
gsk/gsktransform.c:1342: Warning: Gsk: gsk_transform_to_2d: unknown parameter 'm' in documentation comment, should be 'self'
gsk/gsktransform.c:1368: Warning: Gsk: gsk_transform_to_2d: invalid return annotation
gsk/gsktransform.c:1461: Warning: Gsk: gsk_transform_to_translate: unknown parameter 'm' in documentation comment, should be 'self'