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

[Jan Patrick Lehr via cfe-commits]

================
@@ -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
+  target matches the specified architecture. This can be used with the target
+  CPU, triple architecture, or offloading host and device. This is most useful
----------------
jplehr wrote:

```suggestion
  CPU, triple architecture, or offloading host and device. It is most useful
```

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