Try to more clearly specify exactly what the runtool flags do

Author: ehuss

src/doc/rustdoc/src/command-line-arguments.md

@@ -224,14 +224,19 @@ See also `--test`.
 
 ## `--test-runtool`, `--test-runtool-arg`: program to run tests with; args to pass to it
 
+A doctest wrapper program can be specified with the `--test-runtool` flag.
+Rustdoc will execute that wrapper instead of the doctest executable when
+running tests. The first arguments to the wrapper will be any arguments
+specified with the `--test-runtool-arg` flag, followed by the path to the
+doctest executable to run.
+
 Using these options looks like this:
 
 ```bash
-$ rustdoc src/lib.rs --test-runtool runner --test-runtool-arg --do-thing --test-runtool-arg --do-other-thing
+$ rustdoc src/lib.rs --test-runtool path/to/runner --test-runtool-arg --do-thing --test-runtool-arg --do-other-thing
 ```
 
-These options can be used to run the doctest under a program, and also pass arguments to
-that program. For example, if you want to run your doctests under valgrind you might run
+For example, if you want to run your doctests under valgrind you might run:
 
 ```bash
 $ rustdoc src/lib.rs --test-runtool valgrind