docs: Fix stale @doNotGenerate inheritance example (#5002)

[nicowalter256]

Fixes #5002

This PR:

• Splits the section into "hiding a method declared on the class itself"
  (unaffected) and "hiding an inherited method" (only works if the parent
  is fully @doNotGenerate, not just abstract)
• Updates the example to use a @doNotGenerate-annotated parent, which
  still works because no client class is generated for it and there's no
  abstract contract to satisfy
• Adds a warning explaining why an abstract-only parent doesn't support
  this pattern, and that a normal concrete parent still exposes the
  "hidden" method via its own generated client

[developerjamiu]

Thanks for the contribution. LGTM