Tweak Java and C++ test-only documentation.

Bug: 1079253
Change-Id: I30a45ef2eebcf946eecde0d5806478b4a4c3cbfa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2186364Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Auto-Submit: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#766552}

styleguide/c++/c++.md

@@ -34,8 +34,12 @@ features in Chromium is tracked in the separate
 ## Test-only Code
 
   * Functions used only for testing should be restricted to test-only usages
-    with the `ForTesting` suffix. This is checked at presubmit time to ensure
-    these functions are only called by test files.
+    with the testing suffixes supported by
+    [PRESUMBIT.py](https://chromium.googlesource.com/chromium/src/+/master/
+    PRESUBMIT.py).
+    `ForTesting` is the conventional suffix although similar patterns, such as
+    `ForTest`, are also accepted. These suffixes are checked at presubmit time
+    to ensure the functions are called only by test files.
 
 ## Code formatting
 

styleguide/java/java.md

@@ -213,6 +213,15 @@ This is the order of the import groups:
 1. java
 1. javax
 
+## Test-only Code
+Functions used only for testing should be restricted to test-only usages
+with the testing suffixes supported by
+[PRESUMBIT.py](https://chromium.googlesource.com/chromium/src/+/master/
+PRESUBMIT.py).
+`ForTesting` is the conventional suffix although similar patterns, such as
+`ForTest`, are also accepted. These suffixes are checked at presubmit time
+to ensure the functions are called only by test files.
+
 ## Location
 "Top level directories" are defined as directories with a GN file, such as
 [//base](https://chromium.googlesource.com/chromium/src/+/master/base/)