[doc/otbn] Documented instruction count feature.
[doc/otbn] Documented instruction count feature.

Signed-off-by: Vladimir Rozic <832b313c5899de2d6dfc6d8ca4088ea9488a1be3@lowrisc.org>

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