Merge topic 'doc-list-non-existent'

b573a732dc Tests: add tests to check claims in the documentation
b151db01f9 Help: mention non-existent case for list(PREPEND)
b3a249c2cb Help: clarify range for list(INSERT), mention nonexistent / empty case
b6fdcb3df0 Help: clarify description of list(INSERT)
e55f473ea9 Help: clarify that list(APPEND) on a non-existent list creates it

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !6824