gallium/docs: document alpha_to_coverage and alpha_to_one blend state
gallium/docs: document alpha_to_coverage and alpha_to_one blend state

The gallium interface defines these like DX10.  Note that OpenGL ignores
these options if MSAA is disabled or the dest buffer doesn't support
MSAA.

Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler