Improve Nested data type docs

[slvrtrn]

Currently, this entry does not provide a complete tutorial on how to work with Nested.

Few examples:

• The WHERE clause caveats are not covered enough - the fields should be treated as arrays, there is an example, but it still may be not entirely clear to the user, i.e. maybe more in-depth explanation and more examples are required here.
• Missing INSERT example for "For an INSERT query, you should pass all the component column arrays of a nested data structure separately (as if they were individual column arrays). During insertion, the system checks that they have the same length."
• "The ALTER query for elements in a nested data structure has limitations." - it is not clear what limitations to expect, as the article just ends with that.