add `How to overwrite a macro' to the doc [ci skip]

[e-kwsm]

Status

READY/UNDER DEVELOPMENT/FOR DISCUSSION

Description

This PR adds `How to overwrite a macro' subsection (#457 and #463). I'm not a native speaker of English, so feel free to modify it :)

Todos

• [ ] Tests added to cover new/fixed functionality
• [ ] Documentation if necessary
• [ ] Code follows expl3 style guidelines

Minimal example demonstrating the new/fixed functionality

[wspr]

Sorry I missed this at the time. Thanks for the suggestion!