The dolly operators poll function was set to `view3d_rotation_poll`
instead of `view3d_zoom_or_dolly_poll` which reads like a mistake.
As it happens this didn't cause any user visible problems because
RV3D_LOCK_ZOOM_AND_DOLLY is only set when all other locks are set.
Nevertheless, logically the dolly operator should check that dolly
is not locked. Updated the poll function for dolly to check neither
rotation or zoom/dolly is locked with comments noting why both are
needed.
Also expand on code-comments for why dolly enforces perspective view.
7413a8a4f0