Revamp labels documentation

Simplify structure, text, eliminate screenshots
that are no longer needed, add crosslinks, etc.

doc/user/project/issue_board.md

@@ -423,7 +423,7 @@ something between lists by changing a label.
 
 A typical workflow of using the Issue Board would be:
 
-1. You have [created](labels.md#creating-labels) and [prioritized](labels.md#label-priority)
+1. You have [created](labels.md#label-management) and [prioritized](labels.md#label-priority)
    labels so that you can easily categorize your issues.
 1. You have a bunch of issues (ideally labeled).
 1. You visit the Issue Board and start [creating lists](#creating-a-new-list) to

doc/user/search/index.md

@@ -33,8 +33,15 @@ Follow these steps to filter the **Issues** and **Merge Requests** list pages wi
 groups:
 
 1. Click in the field **Search or filter results...**.
-1. In the dropdown menu that appears, select the attribute you wish to filter by (for example,
-   author, assignee, milestone, and so on).
+1. In the dropdown menu that appears, select the attribute you wish to filter by:
+   - Author
+   - Assignee
+   - [Milestone](../project/milestones/index.md)
+   - Release
+   - [Label](../project/labels.md)
+   - My-reaction
+   - Confidential
+   - Search for this text
 1. Select or type the operator to use for filtering the attribute. The following operators are
    available:
    - `=`: Is