Clarify "nested" and "rolling" key sequences

[henrebotha]

Description

This is a minor addition to the docs that clarifies useful terminology that is already referenced. Hopefully it makes understanding dual-role key features a bit better, and makes them easier to talk about.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[stale]

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[henrebotha]

@drashna Could I get a merge?