Add blocking issues feature documentation

Add blocking issues feature documentation and update images

doc/user/project/issues/related_issues.md

@@ -14,6 +14,12 @@ You can relate one issue to another by clicking the related issues "+" button
 in the header of the related issue block. Then, input the issue reference number
 or paste in the full URL of the issue.
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/2035) in GitLab 12.8.
+
+Additionally, you can select whether the current issue relates to, blocks, or is blocked by the issues being entered.
+
+![Adding a related issue](img/related_issues_add_v12_8.png)
+
 Issues of the same project can be specified just by the reference number.
 Issues from a different project require additional information like the
 group and the project name. For example:
@@ -23,9 +29,12 @@ group and the project name. For example:
 - different group: `group/project#44`
 
 Valid references will be added to a temporary list that you can review.
-When ready, click the green "Add related issues" button to submit.
+When you have added all the related issues, click **Add** to submit.
+
+Once you have finished adding all related issues, you will be able to see
+them categorized so their relationships can be better understood visually.
 
-![Adding a related issue](img/related_issues_add.png)
+![Related issue block](img/related_issue_block_v12_8.png)
 
 ## Removing a related issue
 
@@ -33,7 +42,7 @@ In the related issues block, click the "x" icon on the right-side of each issue
 token that you wish to remove. Due to the bi-directional relationship, it
 will no longer appear in either issue.
 
-![Removing a related issue](img/related_issues_remove.png)
+![Removing a related issue](img/related_issues_remove_v12_8.png)
 
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