[clang] Document the const and pure attributes (PR #205881)

[Aaron Ballman via cfe-commits]

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@@ -3157,7 +3157,7 @@ def ArmLocallyStreaming : InheritableAttr, TargetSpecificAttr<TargetAArch64> {
 
 def Pure : InheritableAttr {
   let Spellings = [GCC<"pure">];
-  let Documentation = [Undocumented];
+  let Documentation = [PureDocs];
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AaronBallman wrote:

I waffled back and forth on this but ultimately decided that a combined section makes more sense. I think most people want to know which out of the two to use, and so being able to compare and contrast within one documentation section makes that easier.

https://github.com/llvm/llvm-project/pull/205881