string-list: document iterator behavior on NULL input

string-list: document iterator behavior on NULL input

The for_each_string_list_item() macro takes a string_list and
automatically constructs a for loop to iterate over its contents. This
macro will segfault if the list is non-NULL.

We cannot change the macro to be careful around NULL values because
there are many callers that use the address of a local variable, which
will never be NULL and will cause compile errors with -Werror=address.

For now, leave a documentation comment to try to avoid mistakes in the
future where a caller does not check for a NULL list.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>