CLEANUP: Remove appsession documentation

I was about to partly revert 294d0f08b3d100fcae0e71c26d4f9f93d26e3569,
because there were no 'X' for 'appsession' in the keyword matrix until
I checked the blame, realizing that the feature does not exist any more.

Clearly the documentation is confusing here, the removal note is only
listed *below* the old documentation and the supported sections still
show 'backend' and 'listen'.

It's been 3.5 years and 4 releases (1.6, 1.7, 1.8 and 1.9), I guess
this can be removed from the documentation of future versions.
diff --git a/src/cfgparse-listen.c b/src/cfgparse-listen.c
index 7c64be1..e7cd066 100644
--- a/src/cfgparse-listen.c
+++ b/src/cfgparse-listen.c
@@ -1290,7 +1290,7 @@
 		}
 	}
 	else if (!strcmp(args[0], "appsession")) {  /* cookie name */
-		ha_alert("parsing [%s:%d] : '%s' is not supported anymore, please check the documentation.\n", file, linenum, args[0]);
+		ha_alert("parsing [%s:%d] : '%s' is not supported anymore since HAProxy 1.6.\n", file, linenum, args[0]);
 		err_code |= ERR_ALERT | ERR_FATAL;
 		goto out;
 	}