[otbn, doc] Clarify that OTBN locks up after handling an RMA request
[otbn, doc] Clarify that OTBN locks up after handling an RMA request

This is related to lowRISC/OpenTitan#14995.

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

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