Update documentation about extending Renderer

[mido-app]

Description

• FixesDocs need updating about extending Renderer #1416 update document about extending renderer to be equal behavior between code and docs

Contributor

• Test(s) exist to ensure functionality and minimize regression (if no tests added, list tests covering this PR); or,
• no tests required for this PR.
• If submitting new feature, it has been documented in the appropriate places.

Committer

In most cases, this should be a different person than the contributor.

• Draft GitHub release notes have been updated.
• CI is green (no forced merge required).
• Merge PR

[styfle]

@mido-app Thanks for the PR! 🎉