Werk #7419: Adapt help text to actual state and add log entry

The notificationnumber for hosts and services of custom notifications was silently modified from '0' to '1' since at least June 2015. This was not shown in the help text of the conditions of notifiction rules, and the notify.log.

This werk corrects the help text for the condition "Restrict to nth to mth notification", adds a default of '1' for the minimum value, and adds a log entry for the modification to the notify.log in debug mode.

Rules with a value of '0' for the minimum value will still work, but have to be changed to at least '1' when the rule is saved again.

Note that the "Show / hide notification context" option of the Analyze mode will still show values of '0' since it shows the raw notifications from the core and not the completed context.

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