DOC: config: added optional rst-ttl argument to silent-drop in action lists

This patch adds the optional silent-drop rst-ttl argument to the action lists in
5 places in the configuration manual.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index a4ab08c..7a22216 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -7793,7 +7793,7 @@
     - set-tos <tos>
     - set-var(<var-name>[,<cond> ...]) <expr>
     - set-var-fmt(<var-name>[,<cond> ...]) <fmt>
-    - silent-drop
+    - silent-drop [ rst-ttl <ttl> ]
     - strict-mode { on | off }
     - track-sc0 <key> [table <table>]
     - track-sc1 <key> [table <table>]
@@ -12727,7 +12727,7 @@
     - set-tos <tos>
     - set-var(<var-name>[,<cond> ...]) <expr>
     - set-var-fmt(<var-name>[,<cond> ...]) <fmt>
-    - silent-drop
+    - silent-drop [ rst-ttl <ttl> ]
     - track-sc0 <key> [table <table>]
     - track-sc1 <key> [table <table>]
     - track-sc2 <key> [table <table>]
@@ -12944,7 +12944,7 @@
     - set-tos <tos>
     - set-var(<var-name>[,<cond> ...]) <expr>
     - set-var-fmt(<var-name>[,<cond> ...]) <fmt>
-    - silent-drop
+    - silent-drop [ rst-ttl <ttl> ]
     - switch-mode http [ proto <name> ]
     - track-sc0 <key> [table <table>]
     - track-sc1 <key> [table <table>]
@@ -13334,7 +13334,7 @@
     - set-tos <tos>
     - set-var(<var-name>[,<cond> ...]) <expr>
     - set-var-fmt(<var-name>[,<cond> ...]) <fmt>
-    - silent-drop
+    - silent-drop [ rst-ttl <ttl> ]
     - track-sc0 <key> [table <table>]
     - track-sc1 <key> [table <table>]
     - track-sc2 <key> [table <table>]
@@ -13506,7 +13506,7 @@
     - set-tos <tos>
     - set-var(<var-name>[,<cond> ...]) <expr>
     - set-var-fmt(<var-name>[,<cond> ...]) <fmt>
-    - silent-drop
+    - silent-drop [ rst-ttl <ttl> ]
     - unset-var(<var-name>)
 
   The supported actions are described below.