Docs: move common parts for burndown/burnup charts to a common section

See https://gitlab.com/gitlab-org/gitlab/-/issues/268034Signed-off-by: Daniel Ziegenberg <daniel@ziegenberg.at>

doc/user/project/milestones/burndown_and_burnup_charts.md

@@ -73,10 +73,8 @@ The chart indicates the project's progress throughout that milestone (for issues
 In particular, it shows how many issues were or are still open for a given day in the
 milestone's corresponding period.
 
-The burndown chart can also be toggled to display the cumulative open issue
-weight for a given day. When using this feature, make sure issue weights have
-been properly assigned, since an open issue with no weight adds zero to the
-cumulative value.
+You can also toggle the burndown chart to display the
+[cumulative open issue weight](#switch-between-number-of-issues-and-issue-weight) for a given day.
 
 ### Fixed burndown charts
 
@@ -123,12 +121,21 @@ To view a group's burnup chart:
 ### How burnup charts work
 
 Burnup charts have separate lines for total work and completed work. The total line
-shows when scope is reduced or added to a milestone. The completed work is a count
-of issues closed.
+shows changes to the scope of a milestone. When an open issue is moved to another
+milestone, the "total issues" goes down but the "completed issues" stays the same.
+The completed work is a count of issues closed. When an issue is closed, the "total
+issues" remains the same and "completed issues" goes up.
 
-Burnup charts can show either the total number of issues or total weight for each
-day of the milestone. Use the toggle above the charts to switch between total
-and weight.
+## Switch between number of issues and issue weight
+
+In both burndown or burnup charts you can view them
+either by the total number of issues
+or the total weight for each day of the milestone.
+
+To switch between the two settings, select either **Issues** or **Issue weight** above the charts.
+
+When sorting by weight, make sure all your issues
+have weight assigned, because issues with no weight don't show on the chart.
 
 <!-- ## Troubleshooting
 

doc/user/project/milestones/index.md

@@ -151,10 +151,10 @@ There are also tabs below these that show the following:
 
 ### Burndown Charts
 
-The milestone view contains a [burndown chart](burndown_and_burnup_charts.md),
+The milestone view contains a [burndown and burnup chart](burndown_and_burnup_charts.md),
 showing the progress of completing a milestone.
 
-![burndown chart](img/burndown_chart_v13_6.png)
+![burndown chart](img/burndown_and_burnup_charts_v13_6.png)
 
 ### Milestone sidebar