Fix docs for ignore/only directive differences with compiletest

Author: jieyouxu

README.md

@@ -63,7 +63,9 @@ their command specifies, or the test will fail without even being run.
 
 ## Significant differences to compiletest-rs
 
-* `ignore-target-*` and `only-target-*` operate solely on the triple, instead of supporting things like `macos`
+* `ignore-target-xxx` and `only-target-xxx` requires the `target-` prefix before the `xxx` substring
+  to be matched against target triples, whereas compiletest allows plain `ignore-xxx` without the
+  `target-` prefix.
 * only supports `ui` tests
 * tests are run in named order, so you can prefix slow tests with `0` in order to make them get run first
 * `aux-build`s require specifying nested aux builds explicitly and will not allow you to reference sibling `aux-build`s' artifacts.