Update clang tool documentation.

- Add an easier-to-use command for AST dumping.
- Update caveats to note that it doesn't work for jumbo builds.
- Remove caveat to use git, since it's more-or-less assumed.
- Use reference style links to make the actual text easier to edit.

Change-Id: I25bd7461ccc9fc4910fe0ef18b01e32a37023530
Reviewed-on: https://chromium-review.googlesource.com/734480
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510989}
1 file changed