[lld] [LLD][Docs] Document -z gcs= option in the man page (PR #146522)

[via llvm-commits]

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@@ -959,13 +959,13 @@ disallows overlap.
 x86 only, use shadow stack.
 .Pp
 .It Cm gcs Ns = Ns Ar [implicit|never|always]
-Specify how the GCS bit is set. A set GCS bit indicates that the object file supports the Guarded Control Stack security feature.
+Specifies how the GNU_PROPERTY_AARCH64_FEATURE_1_GCS bit is set in the output ELF file. When set, it indicates support for the Guarded Control Stack (GCS) security feature.
 .Cm implicit
-is the default, where the GCS bit is inferred from the input objects.
+is the (default) GCS feature bit is inferred from the input object marking.
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sivan-shani wrote:

Above is outdated, new version does not suffer from same ambiguity 

https://github.com/llvm/llvm-project/pull/146522