matrix: note that _matrix_multiply can multiply in-place
matrix: note that _matrix_multiply can multiply in-place

This adds a note to clarify that cogl_matrix_multiply allows you to
multiply the @a matrix in-place, so @a can equal @result but @b can't
equal @result.

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