backend: Update docs, add field Key to RequestFuncArgs
backend: Update docs, add field Key to RequestFuncArgs

Key will be used by package esikv to retrieve a value by that key.

SchumacherFM/caddyesi,SchumacherFM/caddyesi,corestoreio/caddy-esi,corestoreio/caddy-esi,corestoreio/caddy-esi,SchumacherFM/caddyesi