MINOR: mux-h2: mention that the mux is compatible with both sides

We declare two configurations for the H2 mux. One supporting only
the frontend in HTTP mode and one supporting both sides in HTX mode.

This is only to ease development at this point. Trying to assign an h2
mux on the server side will still fail during h2_init() anyway instead
of at config parsing time.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index bab4c85..dd74002 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -4369,10 +4369,15 @@
 
 /* PROTO selection : this mux registers PROTO token "h2" */
 static struct mux_proto_list mux_proto_h2 =
-	{ .token = IST("h2"), .mode = PROTO_MODE_HTTP | PROTO_MODE_HTX, .side = PROTO_SIDE_FE, .mux = &h2_ops };
+	{ .token = IST("h2"), .mode = PROTO_MODE_HTTP, .side = PROTO_SIDE_FE, .mux = &h2_ops };
 
 INITCALL1(STG_REGISTER, register_mux_proto, &mux_proto_h2);
 
+static struct mux_proto_list mux_proto_h2_htx =
+	{ .token = IST("h2"), .mode = PROTO_MODE_HTX, .side = PROTO_SIDE_BOTH, .mux = &h2_ops };
+
+INITCALL1(STG_REGISTER, register_mux_proto, &mux_proto_h2_htx);
+
 /* config keyword parsers */
 static struct cfg_kw_list cfg_kws = {ILH, {
 	{ CFG_GLOBAL, "tune.h2.header-table-size",      h2_parse_header_table_size      },