Improve documentation of IpSecTunnelInterface#setUnderlyingNetwork

Clarify the consequence of adding IpSecTunnelInterface to the
underlying network.

Bug: 169855650
Test: builds
Change-Id: I2e3c4fe735b3374b2ff6d23850970e36c0aafda5
Merged-In: I2e3c4fe735b3374b2ff6d23850970e36c0aafda5
1 file changed