plugin/cache: Reword serve_stale docs for clarity (#5394)
plugin/cache: Reword serve_stale docs for clarity (#5394)

* reword for clarity; use consistent terminology

Signed-off-by: Chris O'Haver <c8d69f6e0f4a592980d82af78b0efddff3dcc387@infoblox.com>
chrisohaver/coredns,yongtang/coredns,coredns/coredns