Docs for needs-forced-clang-based-tests (was needs-matching-clang)

Author: Zalathar

src/tests/headers.md

@@ -173,11 +173,12 @@ The following header commands will check LLVM support:
 * `ignore-llvm-version: 7.0 - 9.9.9` — ignores LLVM versions in a range (inclusive)
 * `needs-llvm-components: powerpc` — ignores if the specific LLVM component was not built.
   Note: The test will fail on CI (when `COMPILETEST_REQUIRE_ALL_LLVM_COMPONENTS` is set) if the component does not exist.
-* `needs-matching-clang` — ignores if the version of clang does not match the
-  LLVM version of rustc.
-  These tests are always ignored unless a special environment variable,
-  `RUSTBUILD_FORCE_CLANG_BASED_TESTS`, is set
-  (which is only done in one CI job [`x86_64-gnu-debug`]).
+* `needs-forced-clang-based-tests` —
+  test is ignored unless the environment variable `RUSTBUILD_FORCE_CLANG_BASED_TESTS`
+  is set, which enables building clang alongside LLVM
+  - This is only set in one CI job ([`x86_64-gnu-debug`]), which only runs a tiny
+    subset of `run-make` tests. Other tests with this header will not run at all,
+    which is usually not what you want.
 
 See also [Debuginfo tests](compiletest.md#debuginfo-tests) for headers for
 ignoring debuggers.