Add necessity of ```endif``` statement in the documentation section 6.3

What is the URL of the documentation?

https://plantuml.com/activity-diagram-beta#9d3afea73e90192d

What can be improved?

Point out, that an endif statement is necessary in case the activity does not end with the if else block.

Additional context

I tried to implement something like this, and it took me quite a while until I had figured out, that my code did not work because of the missing endif statement:

@startuml "Missing endif"

start
:A;
split
:B;
split again
if (condition) is (True) then
:C;
else (False)
:D;
'endif /' This will only work, if you uncomment the "endif" '/
end split
end

@endum

plantuml / plantuml

Add necessity of ```endif``` statement in the documentation section 6.3 #1649