What it says is:

If any element of LIST is an lvalue, you can modify it by modifying 
VAR inside the loop. Conversely, if any element of LIST is NOT an 
lvalue, any attempt to modify that element will fail. In other 
words, the foreach loop index variable is an implicit alias 
for each item in the list that you're looping over.

That's referring to the elements of the list, not the list itself. If you're worried, make another array, add new elements onto that, then combine the arrays when it's done.