Adding documentation to getElapsedRealtimeUncertaintyNanos following api council guidance.

Test: N/A comment only
Bug: 129769397
Change-Id: I906494b0b72898149322959f5900626a12d6d86b
(cherry picked from commit d6d4d1efa2084c79161c1ffac303477e9d8e72fe)
1 file changed