Update doc to clarify OWNERS requirements

Clarify that owners of seldom-updated third_party directories
must still be committers.

Change-Id: Ie2903288714d549b407194ee8818e225dd0103a1
Reviewed-on: https://chromium-review.googlesource.com/754381
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514543}
diff --git a/docs/code_reviews.md b/docs/code_reviews.md
index 8a3aa8d..46077d5 100644
--- a/docs/code_reviews.md
+++ b/docs/code_reviews.md
@@ -95,8 +95,9 @@
     discourage people from sending reviews, including writing "slow" or
     "emeritus" after your name.
 
-Seldom-updated directories may have exceptions. Directories in `third_party`
-should list those most familiar with the library.
+Seldom-updated directories may have exceptions to the "substantiality" and
+"recency" requirements. Directories in `third_party` should list those most
+familiar with the library, regardless of how often the code is updated.
 
 ## TBR ("To Be Reviewed")