Update filesystems.md

[mpiercy]

adding more purge policy clarification see ticket https://srcc.freshdesk.com/a/tickets/42941

[kcgthb]

I feel this is detailed in the Expiration policy already?

[mpiercy]

ok, ignore

[vsoch]

I think redundancy doesn't hurt. If there is more information on this in another section and @kcgthb you don't want to have redundancy in the docs, perhaps there should be a link here to that section, since the user might not find the other one?

[kcgthb]

There is one link to the "Expiration policy" section, actually.
But redundancy doesn't hurt indeed, this is a pretty important topic.

[kcgthb]

Change in, thanks @mpiercy!