Update CLA doc

PiperOrigin-RevId: 386882577

Author: Conchylicultor

docs/contribute.md

@@ -72,9 +72,16 @@ issues, you can look at the test discovery logs:
 
 ### Sign the CLA
 
-Ensure you have signed the
-[Contributor License Agreement (CLA)](https://cla.developers.google.com/), so we
-can accept your contributions (only required once for all Google projects).
+Contributions to this project must be accompanied by a Contributor License
+Agreement (CLA). You (or your employer) retain the copyright to your
+contribution; this simply gives us permission to use and redistribute your
+contributions as part of the project. Head over to
+<https://cla.developers.google.com/> to see your current agreements on file or
+to sign a new one.
+
+You generally only need to submit a CLA once, so if you've already submitted one
+(even if it was for a different project), you probably don't need to do it
+again.
 
 ### Follow best practices