Add warning about TelephonyManager Reliability

Add a warning that TelephonyManager methods may not behave
in a consistent way on devices that do not implement
FEATURE_TELEPHONY.

Methods may succeed, may fail loudly, may silently fail, depending
on underlying device support and manufacturer-specific decisions.
When FEATURE_TELEPHONY is not declared, CTS does not enforce any
particular behavior.

Bug: 150006590
Test: compilation (docstring-only change)
Change-Id: I80f5031a43ccc8e5af97d341cf00e48afb80f63a