gallium/docs: document automatic per-sample FS execution

gallium/docs: document automatic per-sample FS execution

Both the GLSL 4.00 specs and DX10.1 specs specify that if a fragment
shader uses the sample ID or sample position inputs, the shader is
automatically run at per sample frequency.  Document that expectation
for gallium fragment shaders.

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