audio: Clarify that MMap h/w position must be continuous

The VTS test already verifies that the MMap hardware position
never resets or goes back, but the official wording in the AIDL
interface was lacking.

Bug: 350998390
Bug: 363139283
Test: none, doc change
Change-Id: I07b9842f53c89549fe648ee839d875d31a7a849d