Make doc list-concatenation more clear as for += and extend().
1. describe `+=` for list-concatenation more accurately
2. add `extend()` example for list-concatenation
3. Fix CI errors for missing helptags reference |+=|
closes: #13983
Signed-off-by: qeatzy <qeatzy@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>