docs: specify default value of edge in Editor.point as start

Clarify in the documentation that the default value for `edge` in `Editor.point` is `start`.
This is helpful for developers who are not deeply familiar with the framework but need to quickly use it for development.
They no longer need to inspect the source code or experiment with sample code to understand the default behavior.

docs/api/nodes/editor.md

@@ -165,7 +165,7 @@ Options: `{depth?: number, edge?: 'start' | 'end'}`
 
 #### `Editor.point(editor: Editor, at: Location, options?) => Point`
 
-Get the start or end point of a location.
+Get the `start` or `end` (default is `start`) point of a location.
 
 Options: `{edge?: 'start' | 'end'}`