make sentence more clear

Author: tshepang

src/tests/compiletest.md

@@ -90,7 +90,8 @@ The following test suites are available, with links for more information:
 | `rustdoc-ui`                         | Check terminal output of `rustdoc` ([see also](ui.md))                   |
 
 Some rustdoc-specific tests can also be found in `ui/rustdoc/`.
-These check rustdoc-related or -specific lints that (also) run as part of `rustc`, not (only) `rustdoc`.
+These tests ensure that lints that are emitted as part of executing rustdoc
+are also run when executing rustc.
 Run-make tests pertaining to rustdoc are typically named `run-make/rustdoc-*/`.
 
 [rustdoc-html-tests]: ../rustdoc-internals/rustdoc-test-suite.md