doc: document open pull request limit

CONTRIBUTING.md

@@ -47,6 +47,8 @@ See [details on our policy on Code of Conduct](./doc/contributing/code-of-conduc
 
 Pull Requests are the way concrete changes are made to the code, documentation,
 dependencies, and tools contained in the `nodejs/node` repository.
+Contributors who are not Collaborators may have no more than 10 pull requests
+open at a time.
 
 * [Dependencies](./doc/contributing/pull-requests.md#dependencies)
 * [Setting up your local environment](./doc/contributing/pull-requests.md#setting-up-your-local-environment)

doc/contributing/first-contributions.md

@@ -65,7 +65,8 @@ make sure to follow the [large pull request guidelines](./large-pull-requests.md
 This gives others the first impression that you are flooding the project.
 It's OK to not get everything right on the first try, but if you open PRs faster
 than they are reviewed, you are wasting an opportunity to improve, and can overwhelm
-both yourself and your reviewers with notifications.
+both yourself and your reviewers with notifications. Contributors who are not
+Collaborators may have no more than 10 pull requests open at a time.
 
 ## Going through the pull request review process
 

doc/contributing/pull-requests.md

@@ -294,6 +294,9 @@ details, but feel free to skip parts if you're not sure what to put.
 If your pull request exceeds 5000 lines of changes, see the
 [large pull requests][] guide for additional requirements.
 
+Contributors who are not Collaborators may have no more than 10 pull requests
+open at a time.
+
 Once opened, pull requests are usually reviewed within a few days.
 
 To get feedback on your proposed change even though it is not ready