Return VPX_CODEC_INCAPABLE on capability failure

Return VPX_CODEC_INCAPABLE on capability failure

All decoder functions should return the VPX_CODEC_INCAPABLE error code
if the algorithm does not have the requested capability.

Move the definitions of VPX_CODEC_CAP_FRAME_THREADING and
VPX_CODEC_CAP_EXTERNAL_FRAME_BUFFER to the VPX_CODEC_CAP_* section.

Change "PUT_SLICE and PUT_FRAME events are posted" to "put_slice and
put_frame callbacks are invoked".

Also fix some other minor comment errors.

This carries back to libvpx the following libaom CL:
https://aomedia-review.googlesource.com/c/aom/+/108405

Change-Id: If67a271c9abbb3eebc2359719cc7d9f235b690d2