Clarifications in the MSv4 description

[dpetry]

As discussed also in person with Jan-Willem Steeb, I think the MSv4 is a good step forward,
but to make the the transition to it easier for long-term users of the MSv2,
it would be good to extend the MSv4 documentation/description further by
(a) clarifying the high-level structure of the MSv4 as a database.
and
(b) providing a "cheat sheet" of corresponding concepts between MSv2 and 4.

Details:
Concerning the MSv4 structure as a database, I would like to see something similar to the following diagram
which I created long ago for the MSv2:

The MSv2 is a relational database made up of tables which are organised in the levels with the MAIN table
and a few others in the top level, the subtables referenced by the top level tables in the second level, and the subtables referenced by the level-2 tables in the third level.
Effectively, the MSv4 must have a similar layout. It would be good to see if it is also organised in three layers
or if there are more.

Concerning the MSv2_to_4 "cheat sheet":
I don't mean a table like the already existing one here:
https://xradio.readthedocs.io/en/latest/measurement_set_overview.html#Translating-between-MS-v2
That table is also somewhat useful, but I would request to fill a table explaining the
correspondence between implementation concepts in the two MS versions:

Here the beginnings of a list of implementation concepts of the MSv2 which should be part of the cheat sheet:

1. casacore Table
2. Table row
3. Table column
4. Table keyword
5. Table field keyword
6. QuantumUnits
7. MEASINFO
8. Array Column
9. Variable (width) Array Column
10. Optional Column
11. Key Column (column which forms part of the key uniquely identifying a Table row)
12. Subtable
13. Optional Subtable
14. Multi-MS (MMS)
15. Virtually concatenated Table
16. Column Storage Manager
    ...

[Jan-Willem]

This is a good idea. When we have created the draft diagrams we will send them to you for comment. Thanks.