It was documented at strange places: in g_sequence_search*() and
g_sequence_lookup*(), but how to insert and sort data is not done by
those functions.

So instead, add the information to the class description (since it
involves several functions), and add also the information in
g_sequence_insert_sorted() and g_sequence_insert_sorted_iter() as a kind
of warning when using those functions.

Note that before this commit, it was not explained *why* it is better to
call g_sequence_sort() after doing a lot of unsorted insertions. Now it
is documented as "more efficient" (I think it's the only reason that
makes sense, otherwise why was it documented?).

https://bugzilla.gnome.org/show_bug.cgi?id=792455