[master] 42ab3c100 Document disuse of SLT_Backend.

[Geoff Simmons]

commit 42ab3c1003e7bf68ddf1e58daf3df55b095a2e8b
Author: Geoff Simmons <geoff at uplex.de>
Date:   Mon Jan 21 10:29:59 2019 +0100

    Document disuse of SLT_Backend.
    
    Closes #2870

diff --git a/include/tbl/vsl_tags.h b/include/tbl/vsl_tags.h
index 3730e7a85..03488cdc8 100644
--- a/include/tbl/vsl_tags.h
+++ b/include/tbl/vsl_tags.h
@@ -160,6 +160,10 @@ SLTM(Backend, 0, "Backend selected",
 	"\t|  |  +- Backend display name\n"
 	"\t|  +---- VCL name\n"
 	"\t+------- Connection file descriptor\n"
+	"\t\n"
+	"\tNOTE: This tag is currently not in use in the Varnish log.\n"
+	"\tIt is mentioned here to document legacy versions of the log,\n"
+	"\tand reserved for possible use in future versions.\n"
 	"\n"
 )