[OTBN] Remove paragraph about unspecified instruction encoding
[OTBN] Remove paragraph about unspecified instruction encoding

The instruction encoding is now documented, remove the paragraph saying
it's not. The instruction alignment isn't really an ISA requirement,
it's an implementation requirement for our instruction fetch unit, it
was out of place in this section.

Fixes #2864

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

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