Correct branch-protection documentation.

The branch-protection codegen option has always been treated as an error
for non-AArch64 targets.

Author: jacobbramley

src/doc/unstable-book/src/compiler-flags/branch-protection.md

@@ -1,7 +1,7 @@
 # `branch-protection`
 
 This option lets you enable branch authentication instructions on AArch64.
-This option is ignored for non-AArch64 architectures.
+This option is only accepted when targeting AArch64 architectures.
 It takes some combination of the following values, separated by a `,`.
 
 - `pac-ret` - Enable pointer authentication for non-leaf functions.