[Clang] [Docs] Add reference to documentation of SysVABIAttr (#85022)

We for some reason already had documentation for this attribute, but just weren’t linking to it.

clang/include/clang/Basic/Attr.td

@@ -2934,7 +2934,7 @@ def Suppress : DeclOrStmtAttr {
 def SysVABI : DeclOrTypeAttr {
   let Spellings = [GCC<"sysv_abi">];
 //  let Subjects = [Function, ObjCMethod];
-  let Documentation = [Undocumented];
+  let Documentation = [SysVABIDocs];
 }
 
 def ThisCall : DeclOrTypeAttr {