graphics: clean up PixelFormat docs

Make the docs more self-contained, not referring to stuff not
defined by the HIDL interfaces.

Mark JPEG, YCBCR_422_888, YCBCR_444_888, FLEX_RGB_888,
FLEX_RGBA_8888 as "MUST NOT USE".  These format are never supposed
to be seen by gralloc and do not belong to the HIDL interfaces.

Note, current.txt change (as opposed to addition) is only allowed
because this is an unreleased interface.

Bug: 32593292
Test: builds
Change-Id: Ia7607cc73dd2f58dfa27e71b0004b21c7ca6904d