[docs] Warn about Jumbo builds for clang-tidy

This change adds a warning to the clang-tidy docs that using a Jumbo
build for Chrome is not supported.

Bug: NONE
Change-Id: I2d714c81689493c35e5e6b4de8d45948f94d8cda
Reviewed-on: https://chromium-review.googlesource.com/1254149Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595471}

docs/clang_tidy.md

@@ -47,7 +47,8 @@ ninja clang-apply-replacements
 ## Running clang-tidy
 
 Running clang-tidy is (hopefully) simple.
-1.  Build chrome normally.\*
+1.  Build chrome normally.\* Note that [Jumbo builds](jumbo.md) are not
+    supported.
 ```
 ninja -C out/Release chrome
 ```
@@ -87,9 +88,9 @@ Copy-Paste Friendly (though you'll still need to stub in the variables):
     chrome/browser
 ```
 
-\*It's not clear which, if any, `gn` flags may cause issues for `clang-tidy`.
-I've had no problems building a component release build, both with and without
-goma. if you run into issues, let us know!
+\*It's not clear which, if any, `gn` flags outside of `use_jumbo_build` may
+cause issues for `clang-tidy`. I've had no problems building a component release
+build, both with and without goma. if you run into issues, let us know!
 
 ## Questions