SpirvShader: Document ops requiring capabilities

... that will not be required for Pastel 1.0.

Bug: b/126873455
Change-Id: I024296dd39ae74613577bd5dbd8d75051b7f4f5c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28790
Tested-by: Ben Clayton <bclayton@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

src/Pipeline/SpirvShader.cpp

@@ -3595,11 +3595,13 @@ namespace sw
 		}
 		case GLSLstd450PackDouble2x32:
 		{
+			// Requires Float64 capability.
 			UNIMPLEMENTED("GLSLstd450PackDouble2x32");
 			break;
 		}
 		case GLSLstd450UnpackDouble2x32:
 		{
+			// Requires Float64 capability.
 			UNIMPLEMENTED("GLSLstd450UnpackDouble2x32");
 			break;
 		}
@@ -3634,16 +3636,19 @@ namespace sw
 		}
 		case GLSLstd450InterpolateAtCentroid:
 		{
+			// Requires sampleRateShading / InterpolationFunction capability.
 			UNIMPLEMENTED("GLSLstd450InterpolateAtCentroid");
 			break;
 		}
 		case GLSLstd450InterpolateAtSample:
 		{
+			// Requires sampleRateShading / InterpolationFunction capability.
 			UNIMPLEMENTED("GLSLstd450InterpolateAtSample");
 			break;
 		}
 		case GLSLstd450InterpolateAtOffset:
 		{
+			// Requires sampleRateShading / InterpolationFunction capability.
 			UNIMPLEMENTED("GLSLstd450InterpolateAtOffset");
 			break;
 		}