Add help doc to timeshift tab explaining options

[oneadvent]

added file config_timeshift.html to accommodate this.

[linniksa]

imho:

1. "unlimited" should be movet upper "Max. *" option or inlines with that (because information is received from left to right and up to bottom)
2. switch (checkbox) logic must be direct - in this case - [x] limit max period - enable text field...

[oneadvent]

Hey @adamsutton do you think number 2 should be done? That changes the backend look at the checkbox so I didn't want to mess with it without your approval. Maybe a separate PR since this one really can't mess anything up and that potentially could.

@Partugal I agree with you on point 1 and this pull request now has that.

[adamsutton]

@oneadvent I have merged you're first commit. I do not agree with changing the order (or inverting the checkbox), its simply not logical. The normal use case I would expect is that users would fill in the limits even if high (this is the safest approach). Also putting "Unlimited" before you've said what its for doesn't make sense. I agree @Partugal that putting it inline would be better (I wanted to do this with some other stuff on the EPG but could never figure it out).

I have removed the size stuff for now (it was never meant to be there) as it doesn't do anything just yet. And I've tidied the text up a bit.