6f5d5db2 and d6d4728f disabled "maintain history on reload" by default,
with the intention to reenable it when we have a better method to
make it discoverable for the user. This was necessary since it became
enabled by default but could be surprising given Geany warned about
losing data before.

This commit tries to resolve the discoverability, by providing an
informational doc message that is shown once to the user, after the first
reload. The doc message also gives the option to disable this feature.