Add helpful note about changed default behaviour

The kind of short .mdp file that might historically have been used for
tests, tutorials or benchmarks used to trigger the group cut-off
scheme up to and including 4.6, because it was either the only
behaviour or the default. Only one year later, the default has changed
to be the Verlet scheme, which might lead to confusion and/or invalid
comparisons. This note will hopefully help people start to understand
any differences.

Refs #1414

Change-Id: I34a2f1c17d4b58e14f81a95c1df3659f13a6cf24