docs: update docs for *authKey, add authKey to HTTP 401 resp body (#7971)

### Describe Your Changes

optimize for
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6226

for user who set `*AuthKey` flag, they will receive new response in
body:
```go
// query arg not set
The provided authKey '' doesn't match -search.resetCacheAuthKey

// incorrect query arg
The provided authKey '5dxd71hsz==' doesn't match -search.resetCacheAuthKey
```

previously, they receive:
```
The provided authKey doesn't match -search.resetCacheAuthKey
```

### Checklist

The following checks are **mandatory**:

- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

lib/httpserver/httpserver.go

@@ -439,6 +439,11 @@ func CheckAuthFlag(w http.ResponseWriter, r *http.Request, expectedKey *flagutil
 	if expectedValue == "" {
 		return CheckBasicAuth(w, r)
 	}
+	if len(r.FormValue("authKey")) == 0 {
+		authKeyRequestErrors.Inc()
+		http.Error(w, fmt.Sprintf("Expected to receive non-empty authKey when -%s is set", expectedKey.Name()), http.StatusUnauthorized)
+		return false
+	}
 	if r.FormValue("authKey") != expectedValue {
 		authKeyRequestErrors.Inc()
 		http.Error(w, fmt.Sprintf("The provided authKey doesn't match -%s", expectedKey.Name()), http.StatusUnauthorized)