[otbn] add comments on bounds of parameters
[otbn] add comments on bounds of parameters

The current implementation of the 384 bit Barrett modular
multiplication expects both input operands to be smaller
than the modulus. This adds comments to point this out.

This is related to #4549.

Signed-off-by: Felix Miller <43d2a12732a784b305469416c0ba5ce1795e752a@gi-de.com>

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