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>

src/gallium/docs/source/tgsi.rst

@@ -3331,6 +3331,9 @@ contains the current sample id (i.e. gl_SampleID) as an unsigned int.
 Only the X component is used.  If per-sample shading is not enabled,
 the result is (0, undef, undef, undef).
 
+Note that if the fragment shader uses this system value, the fragment
+shader is automatically executed at per sample frequency.
+
 TGSI_SEMANTIC_SAMPLEPOS
 """""""""""""""""""""""
 
@@ -3340,6 +3343,9 @@ in the render target (i.e.  gl_SamplePosition) when per-fragment shading
 is in effect.  Position values are in the range [0, 1] where 0.5 is
 the center of the fragment.
 
+Note that if the fragment shader uses this system value, the fragment
+shader is automatically executed at per sample frequency.
+
 TGSI_SEMANTIC_SAMPLEMASK
 """"""""""""""""""""""""