docs: Explain how to switch from nvm to `nodejs_version`, fixes #6204

[syzygy333]

The Issue

Use of nvm and cached version of node can occur if updating config file to use nodejs_version after previously using nvm in post-start hooks.

How This PR Solves The Issue

Adds a note to the nodejs_version documentation with instructions on how to clear the cache.

Tests

No tests necessary.

Related Issue Link(s)

Fixes #6204

Release/Deployment Notes

No special deployment actions needed. Documentation change only.

[rfay]

Thanks!