I agree, that could be better! It would be nice of you to open a feature request with what stuff from docs is missing that you’d like (on GitHub), like this memory stuff.
In general though, you can re-use a single geometry and material (for example) across many meshes, and you’d want to clean up the geometry and material once there are no meshes using it and if they will not be used again (or maybe after a timeout).
Some things, like geometries and materials, get sent to the GPU, so losing a reference to them in JavaScript is not enough to clean them up. They need to be explicitly removed from the GPU when they are not needed any more.
A “How to completely clean up a Three.js scene from the a web app once the scene is no longer needed” guide would be nice for sure. 