Explain "cannot be inherited" for a non-sealed Class.

[WispArchitecture]

Some classes (like ServiceContainer) say, "This class cannot be inherited." in the docs, but the classes are not sealed. Deriving from them seems to work fine. There are several cases. If this is valid, the docs should offer an explanation for this.

[Clockwork-Muse]

They might not be now, but for at least ServiceContainer they used to be (looks like it was unsealed in .NetF 2.0, so quite a long time ago).
This would be a documentation bug, then.