Missing documentation for QMetaContainer

In `qtbase/src/corelib/kernel/qmetacontainer.cpp`, a documentation for the `QMetaSequence` class is provided.

This class inherits from `QMetaContainer`, but no QDoc block-comment with a class topic of `QMetaContainer` is given, such that the autogenerated "inherits" row for the `QMetaSequence` documentation produces a broken link.

As `QMetaContainer` seems to be part of a public interface and `QMetaSequence` depends on it, some documentation for it should be added, when possible.