DOC: ebtree: indicate that prefix insertion/lookup may be used with strings

And indicate what is required for this (that the pattern is properly
terminated by a zero).
(cherry picked from commit c87c93800ce4045b1053302d99a3cd78321a7ec4)
MEDIUM: pattern: use ebtree's longest match to index/lookup string beginning

Being able to map prefixes to values is already used for IPv4/IPv6
but was not yet used with strings. It can be very convenient to map
directories to server farms but large lists may be slow.

By using ebmb_insert_prefix() and ebmb_lookup_longest(), we can
insert strings with their own length as a prefix, and lookup
candidate strings and ensure that the longest matching one will
be returned, which is the longest string matching the entry.