Overload mg_bind to bind to tunnel

PUBLISHED_FROM=f554cc63dfea12455fe5e428c6ce5f3152774f8e

docs/c-api/net.h/struct_mg_bind_opts.md

@@ -10,11 +10,14 @@ signature: |
     struct mg_iface *iface;    /* Interface instance */
   #if MG_ENABLE_SSL
     /* SSL settings. */
-    const char *ssl_cert;    /* Server certificate to present to clients */
+    const char *ssl_cert;    /* Server certificate to present to clients
+                              * Or client certificate to present to tunnel
+                              * dispatcher. */
     const char *ssl_key;     /* Private key corresponding to the certificate.
                                 If ssl_cert is set but ssl_key is not, ssl_cert
                                 is used. */
-    const char *ssl_ca_cert; /* Verify client certificates with this CA bundle */
+    const char *ssl_ca_cert; /* CA bundle used to verify client certificates or
+                              * tunnel dispatchers. */
   #endif
   };
 ---