Add comment for unknown rotation type

Layers with rotation 0/90/180/270 are marked with type ROTATION, layers
with other rotation values (free rotation) are marked as UNKNOWN, added
explanation to prevent doubts about the method

Test: Nothing to test, just added a comment to the code
Change-Id: I4d61ae00cda763c37b5526917e40a9509990a0f4
1 file changed