Clarify prune behavior for different archive contents
Clarify prune behavior for different archive contents

In the online help, explain that archives with different contents need to be
separated via the prefix when pruning to achieve a desired retention policy per
archive set.

Relates to #1824.

raxenak/borg,RonnyPfannschmidt/borg,edgimar/borg,edgewood/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,edgimar/borg,edgewood/borg,raxenak/borg,edgewood/borg,edgimar/borg,RonnyPfannschmidt/borg,RonnyPfannschmidt/borg,raxenak/borg,edgimar/borg,edgewood/borg,raxenak/borg