Add description of union types

[tom-tan]

This request is to solve #716 by clarifying:

• an array such as [int, string] indicates a union type of its elements, and
• its elements may be duplicated.

[mr-c]

@tom-tan Can you fix the test failures?

[codecov]

Codecov Report

Merging #720 (a8a9e1e) into main (b8c7fc3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #720   +/-   ##
=======================================
  Coverage   83.71%   83.71%           
=======================================
  Files          22       22           
  Lines        4507     4507           
  Branches     1244     1244           
=======================================
  Hits         3773     3773           
  Misses        469      469           
  Partials      265      265           

[tom-tan]

The log of the unittests shows that it is already finished successfully but the status is not updated yet...

[tom-tan]

Thanks!