[docs] Document template source headings in aspire new picker

src/frontend/src/content/docs/reference/cli/commands/aspire-new.mdx

@@ -48,6 +48,23 @@ The `--output` option validates the provided path before creating the project:
 
 When running `aspire new` in interactive mode, the command asks one or more questions before generating the project. The available prompts depend on the template and options you provide.
 
+### Template source headings
+
+When you've configured template sources, the template picker groups templates under a heading for each source. Each heading keeps the source's friendly name (or its alias when no name is set) and shows where the templates come from:
+
+- **Git sources** use `Name (repository @ ref)`. When the source doesn't specify a ref, the heading shows `default branch`.
+- **Directory sources** use `Name (path)`.
+
+For example:
+
+```text title="Template picker headings"
+Company templates (https://github.com/example/templates @ release/templates)
+company (https://github.com/example/templates @ default branch)
+Company templates (/work/templates)
+```
+
+Long headings are cropped to fit the terminal width.
+
 ### Use \*.dev.localhost URLs?
 
 During project creation, you may be prompted: