libbinder*: clarify setDataPosition constraints

Documenting constraints on the setDataPosition API. This is how it
is currently used, and adding checks before any write, to validate
objects would be expensive/complex.

Fixes: 232463516
Test: N/A
Change-Id: I5d3c8918e6389d3506ef0b97393be31e4cbe0b70
2 files changed