hashmap: migrate documentation from Documentation/technical into header

While at it, clarify the use of `key`, `keydata`, `entry_or_key` as well
as documenting the new data pointer for the compare function.

Rework the example.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>