[clang] [Clang][Docs] Document -Xarch_ better (PR #127890)

[Joseph Huber via cfe-commits]

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@@ -932,9 +932,18 @@ def W_Joined : Joined<["-"], "W">, Group<W_Group>,
 def Xanalyzer : Separate<["-"], "Xanalyzer">,
   HelpText<"Pass <arg> to the static analyzer">, MetaVarName<"<arg>">,
   Group<StaticAnalyzer_Group>;
-def Xarch__ : JoinedAndSeparate<["-"], "Xarch_">, Flags<[NoXarchOption]>,
-  HelpText<"Pass <arg> to the compiliation if the target matches <arch>">,
-  MetaVarName<"<arch> <arg>">;
+def Xarch__
+    : JoinedAndSeparate<["-"], "Xarch_">,
+      Flags<[NoXarchOption]>,
+      HelpText<"Pass <arg> to the compilation if the target matches <arch>">,
+      DocBrief<
+          [{Specifies that the argument should only be used if the compilation
+  target matches the specified architecture. This can be used with the target
+  CPU, triple architecture, or offloading host and device. It is most useful
+  for separating behavior undesirable on one of the targets when combining many
+  compilation jobs, as is commong with offloading. For example, -Xarch_x86_64,
+  -Xarch_gfx90a, and -Xarch_device are all valid selectors.}]>,
+      MetaVarName<"<arch> <arg>">;
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jhuber6 wrote:

There are special cases? I thought it was self evident for offloading. But I think they are handled as different flags so I could put the help under them if that's what you mean.

https://github.com/llvm/llvm-project/pull/127890