Add documentation for x_bounds, y_bounds

[girvel]

Description

There is no description in docs for Canvas::x_bounds and Canvas::y_bounds, but there is a question about that with response of the author. So this PR puts this response as documentation for the methods with some small corrections and formatting.

Testing guidelines

This PR does not add any code, but all code fragments in documentation are valid & buildable.

Checklist

• [x] I have read the contributing guidelines.
• [x] I have added relevant tests.
• [x] I have documented all new additions.