Document the //@ unused-revision-names: test header (#1991)

Author: Zalathar

Diff for: src/tests/compiletest.md

@@ -639,6 +639,19 @@ Following is classes of tests that support revisions:
 - rustdoc UI tests
 - incremental (these are special in that they inherently cannot be run in parallel)
 
+### Ignoring unused revision names
+
+Normally, revision names mentioned in other headers and error annotations must
+correspond to an actual revision declared in a `revisions` header. This is
+enforced by an `./x test tidy` check.
+
+If a revision name needs to be temporarily removed from the revision list for
+some reason, the above check can be suppressed by adding the revision name to
+an `//@ unused-revision-names:` header instead.
+
+Specifying an unused name of `*` (i.e. `//@ unused-revision-names: *`) will
+permit any unused revision name to be mentioned.
+
 ## Compare modes
 
 Compiletest can be run in different modes, called _compare modes_, which can

Diff for: src/tests/headers.md

@@ -94,6 +94,8 @@ found in [`header.rs`] from the compiletest source.
     * `no-auto-check-cfg` — disable auto check-cfg (only for `--check-cfg` tests)
     * `force-host` — build only for the host target
     * [`revisions`](compiletest.md#revisions) — compile multiple times
+    * [`unused-revision-names`](compiletest.md#ignoring-unused-revision-names) -
+      suppress tidy checks for mentioning unknown revision names
     * [`forbid-output`](compiletest.md#incremental-tests) — incremental cfail rejects output pattern
     * [`should-ice`](compiletest.md#incremental-tests) — incremental cfail should ICE
     * [`known-bug`](ui.md#known-bugs) — indicates that the test is