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

[Jan Patrick Lehr 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 compiliation if the target matches <arch>">,
+      DocBrief<
+          [{Specifies that the argument should only be used if the compileation
----------------
jplehr wrote:

```suggestion
          [{Specifies that the argument should only be used if the compilation
```
I don't know if I made it worse. Look weird to me.

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