comment, and bless, unstable linker flavor test

tests/ui/linkage-attr/unstable-flavor.bpf.stderr

@@ -1,2 +1,2 @@
-error: linker flavor `bpf-linker` is unstable, `-Z unstable-options` flag must also be passed to explicitly use it
+error: the linker flavor `bpf-linker` is unstable, the `-Z unstable-options` flag must also be passed to use the unstable values
 

tests/ui/linkage-attr/unstable-flavor.ptx.stderr

@@ -1,2 +1,2 @@
-error: linker flavor `ptx-linker` is unstable, `-Z unstable-options` flag must also be passed to explicitly use it
+error: the linker flavor `ptx-linker` is unstable, the `-Z unstable-options` flag must also be passed to use the unstable values
 

tests/ui/linkage-attr/unstable-flavor.rs

@@ -1,9 +1,13 @@
+// Even though this test only checks 2 of the 10 or so unstable linker flavors, it exercizes the
+// unique codepath checking all unstable options (see `LinkerFlavorCli::is_unstable` and its
+// caller). If it passes, all the other unstable options are rejected as well.
+//
 // revisions: bpf ptx
 // [bpf] compile-flags: --target=bpfel-unknown-none -C linker-flavor=bpf-linker --crate-type=rlib
-// [bpf] error-pattern: linker flavor `bpf-linker` is unstable, `-Z unstable-options` flag
+// [bpf] error-pattern: linker flavor `bpf-linker` is unstable, the `-Z unstable-options` flag
 // [bpf] needs-llvm-components:
 // [ptx] compile-flags: --target=nvptx64-nvidia-cuda -C linker-flavor=ptx-linker --crate-type=rlib
-// [ptx] error-pattern: linker flavor `ptx-linker` is unstable, `-Z unstable-options` flag
+// [ptx] error-pattern: linker flavor `ptx-linker` is unstable, the `-Z unstable-options` flag
 // [ptx] needs-llvm-components:
 
 #![feature(no_core)]