Add internal docs for metadata and array behind Extended help wrappers

[felixcremer]

This adds docstrings to internal functions in metadata and array which are hidden behind an Extended help layer. The idea is to provide the docstring, but to make clear that the functions are internal.
The docstrings could clearly be improved, they are based on my current understanding from reading the code.
If you like the overall look of this, I might be going through more of the internal functions in the next weeks to add more internal documentation.

I also added docstrings to DimVector and similar types following the docstring examples for AbstractVector.
In the REPL this would look like this for internal functions:

help?> DD._dimlength
  Internal function

  ─────────────────────────────────────────────────────────────────────────

Extended help is available with `??`

help?> ?DD._dimlength
  Internal function

  Extended help
  ≡≡≡≡≡≡≡≡≡≡≡≡≡

  _dimlength(dim)

  Return the length of the dimension dim or in case of a Tuple the length
  of every dimension in the Tuple.