docs: add warning to backend/swift about auto-expire
docs: add warning to backend/swift about auto-expire

If the user uses the auto-expire value in the backend/swift settings
then swift will automatically delete their Statefile which is likely
something the user doesn't want given how Terraform works.

pradeepbhadani/terraform,drebes/terraform,drebes/terraform,pdecat/terraform,ewbankkit/terraform,apparentlymart/terraform,rnaveiras/terraform,pryorda/terraform,brandontosch/terraform,iljaskevic/terraform,nlamirault/terraform,heimweh/terraform,rnaveiras/terraform,pdecat/terraform,pryorda/terraform,pdecat/terraform,luis-silva/terraform,drebes/terraform,heimweh/terraform,mattray/terraform,youhong316/terraform,luis-silva/terraform,VladRassokhin/terraform,VladRassokhin/terraform,mattray/terraform,iljaskevic/terraform,tommynsong/terraform,nlamirault/terraform,rnaveiras/terraform,hashicorp/terraform,brandontosch/terraform,apparentlymart/terraform,apparentlymart/terraform,youhong316/terraform,hashicorp/terraform,gechr/terraform,pryorda/terraform,ewbankkit/terraform,mattray/terraform,luis-silva/terraform,iljaskevic/terraform,youhong316/terraform,mattray/terraform,tommynsong/terraform,apparentlymart/terraform,pradeepbhadani/terraform,youhong316/terraform,ewbankkit/terraform,gechr/terraform,iljaskevic/terraform,tommynsong/terraform,nlamirault/terraform,brandontosch/terraform,heimweh/terraform,VladRassokhin/terraform,ewbankkit/terraform,gechr/terraform,pdecat/terraform,heimweh/terraform,luis-silva/terraform,brandontosch/terraform,apparentlymart/terraform,heimweh/terraform,VladRassokhin/terraform,pradeepbhadani/terraform,brandontosch/terraform,hashicorp/terraform,drebes/terraform