Make mg_http_check_digest_auth() public

docs/c-api/http.h/items.json

docs/c-api/http.h/mg_http_check_digest_auth.md

docs/coap.adoc

docs/common.adoc

docs/dns-server.adoc

docs/dns.adoc

docs/http.adoc

docs/index-docinfo.html

docs/index.html

docs/internal.adoc

docs/json-rpc.adoc

docs/mbuf.adoc

docs/mqtt-broker.adoc

docs/mqtt.adoc

docs/net.adoc

docs/net_if.adoc

docs/resolv.adoc

docs/uri.adoc

docs/util.adoc

mongoose.c