Clarify Acquire/Release behavior for external memory specs
to call out the scope of operations as well as
the behavior in case of multiple acquire/release calls.

Fixes #1078, #1086

Address review comments on PR#1176

Fixes #1078, #1086

Address some of the comments that were
left out in earlier update.

Fix the typo suggested by Kevin to replace aquired to acquired.

Co-authored-by: Kévin Petit <kevin.petit@arm.com>