Document `TraitType.read_only`

By searching the repo, I was able to find this description in the changelog:

> Trait attributes initialized with read_only=True can only be set with the set_trait method. Attempts to directly modify a read-only trait attribute raises a TraitError.

This description would be nice to have in docs/docstrings!

Search the repository for TraitType.read_only and the changelog description, then inspect the relevant docs or docstrings. Document that read-only trait attributes can only be set with set_trait and that direct modification raises TraitError; done when this behavior is covered where TraitType is documented.

Written by the indexing model from the issue text.