i420_rgb,i420_yuy2,i422_yuy2: fix and clarify SIMD documentation

i420_rgb,i420_yuy2,i422_yuy2: fix and clarify SIMD documentation

** documentation fixes only! **

includes...

SSE:
 - Fixes stated Y/Cb/Cr counts of 8:4:4 to 16:8:8 (SSE2 is 128 bit, those
   were the counts from the MMX version)
 - Adjusts byte layout per being 16 bytes, for correctness and clarity
 - Fixes mistakes in the latter lines of SSE2_UNPACK_32_ABGR_UNALIGNED
   and SSE2_UNPACK_32_ABGR_ALIGNED of i420_rgb
 - Fixes swapped G and B in comments of store lines in
   SSE2_UNPACK_32_ARGB_ALIGNED and SSE2_UNPACK_32_ARGB_UNALIGNED in same
 - Adds note to clarify that 15/16 bit unpacking is showing bit layout for
   much of the documentation, whereas most is byte layout, which left me
   confused for a while, and clarified the conversion comment (nothing to
   do with RGB24 format as could be confused) (i420_rgb)
 - Fixed position of red bits in conversion for RV15, which seem to have
   ignored the right shift done to xmm1 earlier
 - Fixed "pixels 4-7" -> "pixels 8-15"
MMX (i420_rgb only)
 - Fixed alignment
 - Added 15/16 bit note about bit layout and clarified conversion as above
 - Fixed "pixels 0-3" -> "pixels 4-7"
 - Fixed position of red bits in conversion for RV15, as above
 - Used zeros instead of underscores in byte layout for consistency

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>