man: say more explicitly what to do in the SD_JOURNAL_INVALIDATE event

This rewords the section, explicitly distuingishing the cases of clients
that only want a continious log stream (which can simply treat
SD_JOURNAL_INVALIDATE the same way as SD_JOURNAL_APPEND) and those which
want to represent on screen the full state of the log data on disk.

This is an alternative to a part of PR #9060, but keeps an explanation
of the destinction of handling depending on the type of client.

Fixes: #8963

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