doh-insecure.md: expand

Closes #13889

docs/cmdline-opts/doh-insecure.md

@@ -8,10 +8,20 @@ Category: dns tls
 Multi: boolean
 See-also:
   - doh-url
+  - insecure
+  - proxy-insecure
 Example:
   - --doh-insecure --doh-url https://doh.example $URL
 ---
 
 # `--doh-insecure`
 
-Same as --insecure but used for DoH (DNS-over-HTTPS).
+By default, every connection curl makes to a DoH server is verified to be
+secure before the transfer takes place. This option tells curl to skip the
+verification step and proceed without checking.
+
+**WARNING**: using this option makes the DoH transfer and name resolution
+insecure.
+
+This option is equivalent to --insecure and --proxy-insecure but used for DoH
+(DNS-over-HTTPS) only.