Improve documentation of non configurable rate limits

doc/security/rate_limits.md

@@ -89,7 +89,7 @@ The **rate limit** is 5 requests per minute per user.
 
 ### Users sign up
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77835) in GitLab 14.7.
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/339151) in GitLab 14.7.
 
 There is a rate limit per IP address on the `/users/sign_up` endpoint. This is to mitigate attempts to misuse the endpoint. For example, to mass
 discover usernames or email addresses in use. 
@@ -98,19 +98,19 @@ The **rate limit** is 20 calls per minute per IP address.
 
 ### Update username
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77221) in GitLab 14.7.
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/339152) in GitLab 14.7.
 
-There is a rate limit on the update username action. This is enforced to mitigate misuse of the feature. For example, to mass discover
+There is a rate limit on how frequently a username can be changed. This is enforced to mitigate misuse of the feature. For example, to mass discover
 which usernames are in use.
 
 The **rate limit** is 10 calls per minute per signed-in user.
 
 ### Username exists
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77119) in GitLab 14.7.
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/29040) in GitLab 14.7.
 
-There is a rate limit for the internal endpoint `/users/:username/exists`, used by registration to perform a client-side validation for
-uniqueness of the chosen username. This is to mitigate the risk of misuses, such as mass discovery of usernames in use.
+There is a rate limit for the internal endpoint `/users/:username/exists`, used upon sign up to check if a chosen username has already been taken.
+This is to mitigate the risk of misuses, such as mass discovery of usernames in use.
 
 The **rate limit** is 20 calls per minute per IP address.