Update configuration comment about LOWBITDEPTH
Update configuration comment about LOWBITDEPTH

The comment was misleading as the codec always supports 8-bit,
regardless of the value of CONFIG_LOWBITDEPTH.
This flag just enables the optimized-for-8-bits pipeline,
without changing the actual YUV output.

Change-Id: Ic2f041870acf4e2ee435021aa42e8f013ef52b78

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom