Describe the logic of event handler conditions in more detail
Describe the logic of event handler conditions in more detail

Due to historical reasons, since *one* event handlers can be
attached to several events, the conditions for
serial, tokenrealm, tokenresolver and user_token_number behave
a bit differently.
The conditions also evaluate to true, if the needed information is
not available - aka. no serial, no tokenrealm, no tokenresolver or
no user.

Working on #1460

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