Update structure per request

Adds section, not separate table

doc/ci/variables/README.md

@@ -81,6 +81,8 @@ echo "$KUBE_CA_PEM" > "$(pwd)/kube.ca.pem"
 kubectl config set-cluster e2e --server="$KUBE_URL" --certificate-authority="$(pwd)/kube.ca.pem"
 ```
 
+There are [some predefined variables](#custom-variables-validated-by-gitlab) of this type, which may be further validated. They will appear when you add or update a variable.
+
 ##### File type
 
 The example above can now be simplified by creating a "File" type variable, and using
@@ -116,6 +118,21 @@ If the value does not meet the requirements above, then the CI variable will fai
 In order to save, either alter the value to meet the masking requirements
 or disable **Masked** for the variable.
 
+#### Custom variables validated by GitLab
+
+Some variables are listed in the UI so you can choose them more quickly.
+GitLab validates the values of these variables to ensure they are in the correct format.
+
+| Variable                | Allowed Values                                     | Introduced in |
+|-------------------------|----------------------------------------------------|---------------|
+| `AWS_ACCESS_KEY_ID`     | 20 characters: letters, digits                     | 12.10         |
+| `AWS_DEFAULT_REGION`    | Any                                                | 12.10         |
+| `AWS_SECRET_ACCESS_KEY` | 40 characters: letters, digits, special characters | 12.10         |
+
+NOTE: **Note:**
+When you store credentials, there are security implications. If you are using AWS keys,
+for example, follow their [best practices](https://docs.aws.amazon.com/general/latest/gr/aws-access-keys-best-practices.html).
+
 ## Getting started
 
 To get started with environment variables in the scope of GitLab