[otbn, doc] Document missed grant fatal error
[otbn, doc] Document missed grant fatal error

We raise a BAD_INTERNAL_STATE fatal error where an imem or dmem request
isn't granted.

Signed-off-by: Greg Chadwick <3e4c3d6d87c017d1a2fa161477653b4039c6b2fb@lowrisc.org>

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