Clarify durations in NPC.md

Also adds examples.

doc/NPCs.md

@@ -545,9 +545,10 @@ The NPC will train your character in a skill or martial art.
 #### companion_mission: role_string
 The NPC will offer you a list of missions for your allied NPCs, depending on the NPC's role.
 
-#### u_add_effect: effect_string, (optional duration: duration_string)
-#### npc_add_effect: effect_string, (optional duration: duration_string)
-Your character or the NPC will gain the effect for `duration_string` turns.
+#### u_add_effect: effect_string, duration: duration_integer or PERMANENT
+#### npc_add_effect: effect_string, duration: duration_integer or PERMANENT
+Your character or the NPC will gain the effect for `duration_integer` turns.
+NOTE: `duration_integer` should not be enclosed in quotation marks.
 
 #### u_add_trait: trait_string
 #### npc_add_trait: trait_string
@@ -726,6 +727,16 @@ npc_service cash available.  Useful to check if the player character can hire an
   "topic": "TALK_NONE",
   "condition": { "npc_has_effect": "infected" }
 },
+{
+  "text": "I'm sorry for offending you.  I predict you will feel better in exactly one hour.",
+  "topic": "TALK_NONE",
+  "effect": { "npc_add_effect": "deeply_offended", "duration": 600 }
+},
+{
+  "text": "Nice to meet you too.",
+  "topic": "TALK_NONE",
+  "effect": { "u_add_effect": "has_met_example_NPC", "duration": "PERMANENT" }
+},
 {
   "text": "[INT 11] I'm sure I can organize salvage operations to increase the bounty scavengers bring in!",
   "topic": "TALK_EVAC_MERCHANT_NO",