fix(nicknames): register the tab title for localization

The General > Nicknames tab title used L["Nicknames"] but was never registered in the enUS locale, so it could not be translated. Also drop a stale /dfnick comment in the options panel.

Author: Maelareth

Locales/enUS.lua

@@ -1903,6 +1903,7 @@ L["Marker style"] = true
 L["Match"] = true
 L["My added"] = true
 L["Nickname"] = true
+L["Nicknames"] = true
 L["No B.net friends found."] = true
 L["No characters found."] = true
 L["No group members found."] = true

Options/NicknamesPage.lua

@@ -770,7 +770,7 @@ function DF.BuildNicknamesPage(guiRef, pageRef, dbRef)
         recvContent:SetHeight(mmax(1, n * ROW_HEIGHT))
     end
 
-    -- Keep this panel in sync with changes made anywhere (e.g. /dfnick).
+    -- Keep this panel in sync with changes made anywhere (e.g. incoming shares).
     NK.onChange = Refresh
     Refresh()
 end