Use eddyb's suggestion from rust-lang#11 about logging

Author: mark-i-m

src/compiler-debugging.md

@@ -162,9 +162,10 @@ your log filter, e.g. to get the logs for a specific module, you can run the
 compiler as `RUST_LOG=module::path rustc my-file.rs`. All `debug!` output will
 then appear in standard error.
 
-Note that unless you use a very strict filter, the logger will emit a *lot*
-of output - so it's typically a good idea to pipe standard error to a file
-and look at the log output with a text editor.
+**Note that unless you use a very strict filter, the logger will emit a lot of
+output, so use the most specific module(s) you can (comma-separated if
+multiple)**. It's typically a good idea to pipe standard error to a file and
+look at the log output with a text editor. You should also .
 
 So to put it together.