Expand Outline Command Documentation

This is a request to expand the [outline command documentation](https://github.com/aseprite/api/blob/main/api/command/Outline.md). For context, please see this [Aseprite community thread](https://community.aseprite.org/t/apply-outline-of-selected-pattern-and-colors-to-all-layers-or-frames-solved/26523). The matrix parameter also accepts an integer, where each neighboring pixel is given an integer code. The codes can be composited together with bitwise inclusive or.

Open api/command/Outline.md and compare the current matrix parameter documentation with the linked Aseprite community thread. Clarify that the parameter accepts an integer, explain the neighboring-pixel codes and their bitwise inclusive-or composition using the available context, and verify the resulting documentation is complete and understandable.

Written by the indexing model from the issue text.