Add documentation for ProgressBar text customisation. https://github.com/AvaloniaUI/Avalonia/pull/8454

docs/controls/progressbar.md

@@ -2,6 +2,26 @@
 
 The `ProgressBar` control allow for showing dynamic progress status.
 
+### Customising the progress text
+
+When [`ShowProgressText`](http://reference.avaloniaui.net/api/Avalonia.Controls/ProgressBar/590A8B3E) is `true`, text on the progress bar will be displayed.
+
+![Basic Progress Bar](<../../.gitbook/assets/progressbar (1).png>)
+
+By default this text shows the percentage completion, according to the [`Value`](http://reference.avaloniaui.net/api/Avalonia.Controls.Primitives/RangeBase/E111DF5B), [`Minimum`](http://reference.avaloniaui.net/api/Avalonia.Controls.Primitives/RangeBase/8F9BD1EA) and [`Maximum`](http://reference.avaloniaui.net/api/Avalonia.Controls.Primitives/RangeBase/C07B22E9). The format of this text can be customised by using the `ProgressTextFormat` property. This expects a string which will be passed to a [`string.Format`](https://docs.microsoft.com/en-us/dotnet/api/system.string.format#system-string-format(system-string-system-object())) call with the value of `ProgressTextFormat` as the format string. The following format items are available at the given indices:
+
+* 0 = Value
+* 1 = Value as a Percentage from 0 to 100 (e.g. `Minimum = 0`, `Maximum = 50`, `Value = 25`, then `Percentage = 50`)
+* 2 = Minimum
+* 3 = Maximum
+
+#### Progress Text Format Example
+
+|Min |Max |Value |Format String                    |Output                       |
+|----|----|------|---------------------------------|-----------------------------|
+|0   |20  |17    |`{0}/{3} Tasks Complete ({1:0}%)`|`17/20 Tasks Complete (85%)` |
+
+
 ### Reference <a id="reference"></a>
 
 [ProgressBar](http://reference.avaloniaui.net/api/Avalonia.Controls/ProgressBar/)
@@ -9,4 +29,3 @@ The `ProgressBar` control allow for showing dynamic progress status.
 ### Source code <a id="source-code"></a>
 
 [ProgressBar.cs](https://github.com/AvaloniaUI/Avalonia/blob/master/src/Avalonia.Controls/ProgressBar.cs)
-