Docs: Add suggested copy-edits for x509 rake task

doc/raketasks/x509_signatures.md

 # X.509 signatures **(CORE ONLY)**
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/122159) in GitLab 12.10.
+
 When [signing commits with X.509](../user/project/repository/x509_signed_commits/index.md),
 the trust anchor might change and the signatures stored within the database must be updated.
 
@@ -10,13 +12,13 @@ certificate store.
 
 To update all X.509 signatures, run:
 
-**Omnibus Installation**
+**Omnibus Installations:**
 
 ```shell
 sudo gitlab-rake gitlab:x509:update_signatures
 ```
 
-**Source Installation**
+**Source Installations:**
 
 ```shell
 sudo -u git -H bundle exec rake gitlab:x509:update_signatures RAILS_ENV=production