Add a note about checks with multiple revisions (rust-lang#1963)

Author: tgross35

src/tests/compiletest.md

@@ -593,8 +593,7 @@ You can therefore use `#[cfg(foo)]` etc within the test to tweak
 each of these results.
 
 You can also customize headers and expected error messages to a particular
-revision. To do this, add `[foo]` (or `bar`, `baz`, etc) after the `//`
-comment, like so:
+revision. To do this, add `[revision-name]` after the `//` comment, like so:
 
 ```rust,ignore
 // A flag to pass in only for cfg `foo`:
@@ -606,6 +605,9 @@ fn test_foo() {
 }
 ```
 
+Multiple revisions can be specified in a comma-separated list, such as
+`//[foo,bar,baz]~^`.
+
 In test suites that use the LLVM [FileCheck] tool, the current revision name is
 also registered as an additional prefix for FileCheck directives: