include: sys: crc: Clarify documentation of two crc functions

The crc16_ccitt and crc16_itu_t functions' documentation could lead to
slight misunderstandings because of the fact that, although they bear
those names, those functions are actually able to compute CRCs that bear
other names, depending on the inital seed provided and an optional XOR
on the result.
In order to avoid confusion, document thoroughly with links to the
standard reveng page that lists all existing CRCs.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>