Documented :define_alias_split().

Such documentation is now quite detailed. We also attempted to implement
:define_alias_split_block(), but frankly failed (due to unforseen intricacies in
the zsh alias parser). That said, we believe we've documented a means of
reimplementing such function correctly.

Also defined a new :define_func_if_pathable_in_list_order_or_noop() function
split from :define_func_if_pathable_or_noop(), the latter of which no longer
accepts an optional argument. ("Huzzah!", the whingy elves did croon.)