[sw,crypto] Add header comment to cryptolib  API.
[sw,crypto] Add header comment to cryptolib  API.

Clarifies that the API as presented here is a work in progress and
subject to change.

Signed-off-by: Jade Philipoom <23b156e860ef6ab36b055884a79031aa7723874a@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan