[cogl-matrix] Documents that CoglMatrix members should be considered read only

In the future if we want to annotate matrices with internal flags, and add
caching of the inverse matrix then we need to ensure that all matrix
modifications are done by cogl_matrix API so we'd know when to dirty the
cache or update the flags.

This just adds documentation to that effect, and assuming the most likley
case where someone would try and directly write to matrix members would
probably be to load a constant matrix other than the identity matrix; I
renamed cogl_matrix_init_from_gl_matrix to cogl_matrix_init_from_array to
make it seem more general purpose.

doc/reference/cogl/cogl-sections.txt

@@ -322,6 +322,8 @@ cogl_matrix_multiply
 cogl_matrix_rotate
 cogl_matrix_translate
 cogl_matrix_scale
+cogl_matrix_init_from_array
+cogl_matrix_get_array
 </SECTION>
 
 <SECTION>