docs: Set llvmpipe and softpipe note only for MSAA.
docs: Set llvmpipe and softpipe note only for MSAA.

Right now, in mesamatrix.net, the footnote is set so that it seems to be
for all the features, while actually it only applies to MSAA.

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler