Refactor GPG signing docs

doc/user/project/gpg_signed_commits/index.md

 # Signing commits with GPG
 
+> [Introduced][ce-9546] in GitLab 9.5.
+
+GitLab can show whether a commit is verified or not when signed with a GPG key.
+All you need to do is upload the public GPG key in your profile settings.
+
+GPG verified tags are not supported yet.
+
 ## Getting started
 
+Here are a few guides to get you started with GPG:
+
 - [Git Tools - Signing Your Work](https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work)
-- [Git Tools - Signing Your Work: GPG introduction](https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work#_gpg_introduction)
-- [Git Tools - Signing Your Work: Signing commits](https://git-scm.com/book/en/v2/Git-Tools-Signing-Your-Work#_signing_commits)
+- [Managing OpenPGP Keys](https://riseup.net/en/security/message-security/openpgp/gpg-keys)
+- [OpenPGP Best Practices](https://riseup.net/en/security/message-security/openpgp/best-practices)
+- [Creating a new GPG key with subkeys](https://www.void.gr/kargig/blog/2013/12/02/creating-a-new-gpg-key-with-subkeys/) (advanced)
 
 ## How GitLab handles GPG
 
@@ -12,24 +22,26 @@ GitLab uses its own keyring to verify the GPG signature. It does not access any
 public key server.
 
 In order to have a commit verified on GitLab the corresponding public key needs
-to be uploaded to GitLab.
+to be uploaded to GitLab. For a signature to be verified two prerequisites need
+to be met:
 
-For a signature to be verified two prerequisites need to be met:
-
-1. The public key needs to be added to GitLab
+1. The public key needs to be added your GitLab account
 1. One of the emails in the GPG key matches your **primary** email
 
-## Add a GPG key
+## Adding a GPG key
 
-1. On the upper right corner, click on your avatar and go to your **Settings**.
+>**Note:**
+Once you add a key, you cannot edit it, only remove it. In case the paste
+didn't work, you'll have to remove the offending key and re-add it.
 
-    ![Settings dropdown](../../../gitlab-basics/img/profile_settings.png)
+You can add a GPG key in your profile's settings:
 
-1. Navigate to the **GPG keys** tab.
+1. On the upper right corner, click on your avatar and go to your **Settings**.
 
-    ![GPG Keys](img/profile_settings_gpg_keys.png)
+    ![Settings dropdown](../../profile/img/profile_settings_dropdown.png)
 
-1. Paste your **public** key in the 'Key' box.
+1. Navigate to the **GPG keys** tab and paste your _public_ key in the 'Key'
+   box.
 
     ![Paste GPG public key](img/profile_settings_gpg_keys_paste_pub.png)
 
@@ -38,47 +50,44 @@ For a signature to be verified two prerequisites need to be met:
 
     ![GPG key single page](img/profile_settings_gpg_keys_single_key.png)
 
->**Note:**
-Once you add a key, you cannot edit it, only remove it. In case the paste
-didn't work, you will have to remove the offending key and re-add it.
-
-## Remove a GPG key
-
-1. On the upper right corner, click on your avatar and go to your **Settings**.
-
-1. Navigate to the **GPG keys** tab.
-
-1. Click on the trash icon besides the GPG key you want to delete.
+## Removing a GPG key
 
->**Note:**
 Removing a key **does not unverify** already signed commits. Commits that were
 verified by using this key will stay verified. Only unpushed commits will stay
-unverified once you remove this key.
+unverified once you remove this key. To unverify already signed commits, you need
+to [revoke the associated GPG key](#revoking-a-gpg-key) from your account.
 
-## Revoke a GPG key
+To remove a GPG key from your account:
 
 1. On the upper right corner, click on your avatar and go to your **Settings**.
-
 1. Navigate to the **GPG keys** tab.
+1. Click on the trash icon besides the GPG key you want to delete.
 
-1. Click on **Revoke** besides the GPG key you want to delete.
+## Revoking a GPG key
 
->**Note:**
 Revoking a key **unverifies** already signed commits. Commits that were
 verified by using this key will change to an unverified state. Future commits
 will also stay unverified once you revoke this key. This action should be used
 in case your key has been compromised.
 
+To revoke a GPG key:
+
+1. On the upper right corner, click on your avatar and go to your **Settings**.
+1. Navigate to the **GPG keys** tab.
+1. Click on **Revoke** besides the GPG key you want to delete.
+
 ## Verifying commits
 
-1. Within a project navigate to the **Commits** tag. Signed commits will show a
+1. Within a project navigate to the **Commits** tab. Signed commits will show a
    badge containing either "Verified" or "Unverified", depending on the
    verification status of the GPG signature.
 
     ![Signed and unsigned commits](img/project_signed_and_unsigned_commits.png)
 
-1. By clicking on the GPG badge details of the signature are displayed.
+1. By clicking on the GPG badge, details of the signature are displayed.
 
     ![Signed commit with verified signature](img/project_signed_commit_verified_signature.png)
 
     ![Signed commit with verified signature](img/project_signed_commit_unverified_signature.png)
+
+[ce-9546]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9546