lte: Remove assert upon RAR timeout in RLF example and..

This merge request introduces the following two changes:

1. It removes an assert, which I forgot to remove while testing RAR timeouts.
2. It updates the documentation of the example to improve the understanding of the behavior caused by RLF in Scenario A (https://www.nsnam.org/docs/models/html/lte-user.html#lena-radio-link-failure-one-enb)