Add a SLT_Notice VSL tag

This VSL tag will be used for informational messages related to
exceptional handling of requests.

doc/sphinx/reference/vsl.rst

@@ -102,6 +102,17 @@ Error
 	Backend request failed to vcl_backend_error.
 
 
+NOTICE MESSAGES
+===============
+
+Notice messages contain informational messages about the handling of a
+request. These can be exceptional circumstances encountered that causes
+deviation from the normal handling. The messages are prefixed with [core]
+for core Varnish generated messages, or with [<VMOD name>] for VMOD
+generated messages. The [core] messages are described in detail below, see
+the individual VMOD documentation for messages from VMODs.
+
+
 HISTORY
 =======
 

include/tbl/vsl_tags.h

@@ -676,6 +676,13 @@ SLTM(VCL_use, 0, "VCL in use",
 	"\n"
 )
 
+SLTM(Notice, 0, "Informational messages about request handling",
+	"Informational log messages on events occured during request"
+	" handling. Lines are prefixed with either [core] or [<VMOD name>]."
+	" See the NOTICE MESSAGES section below or the individual VMOD manual"
+	" pages for detailed information of notice messages.\n"
+)
+
 #undef NOSUP_NOTICE
 #undef NODEF_NOTICE
 #undef SLTM