docs:added information about trailing_commas and all its possibles v… (#7121)

fixes #6642 

Added trailing_commas subsection with:
Description of both available values (automate and preserve)
Detailed explanation of each value's behavior (matching the text from
the GitHub PR mentioned in the issue)
Example YAML configuration showing both options together.

This addresses the Issue #6642 by documenting the trailing_commas option
and its possible values, making it clear to users how to configure this
formatter behavior.

@parlough @conooi @antfitch  please review

Author: Anushreebasics

src/content/tools/analysis.md

@@ -645,11 +645,38 @@ analyzer:
 ## Configuring `dart format`
 
 You can configure the behavior of [`dart format`][] by adding a
-`formatter` section to the analysis options file
-specifying your preferred `page_width`.
+`formatter` section to the analysis options file.
+
+### Configurable options
+
+#### `page_width`
+
+You can specify your preferred `page_width` (line length).
+The default is 80 characters.
 
 For more information, read [Configuring formatter page width][].
 
+#### `trailing_commas`
+
+You can configure how the formatter handles trailing commas
+in argument and parameter lists
+using the `trailing_commas` option.
+It accepts one of two values:
+
+`automate` (default)
+: The formatter adds and removes trailing commas based on
+  its decision to split the surrounding construct.
+
+`preserve`
+: A trailing comma forces the surrounding construct to split.
+  The formatter adds a trailing comma when it splits a construct but does not remove one.
+
+```yaml title="analysis_options.yaml"
+formatter:
+  page_width: 120
+  trailing_commas: preserve
+```
+
 ## Resources
 
 Use the following resources to learn more about static analysis in Dart: