Provide escape item in water-surrounded vaults

Water vaults are one of the few places that can/will generate completely
sealed off in a normal level.  Other such spots are designed to provide
a guaranteed means of escape (vault guard, scrolls of teleportation in
niches, etc) -- water vaults were an exception that didn't do this, so a
hero who fell into one from above could have ended up in a position
where she had no choice but to wait to starve to death or #quit. Provide
an escape item in one of the vault's chests to give a hero more options
in that position.

Also fix a minor mistake (I'm pretty sure, though I'm not a Lua expert
enough to be certain) in an nhlib.c comment describing how to use
obj.addcontent() -- when called as box.addcontent(contents) as the
comment suggested it produces an error, but works OK when called as
box:addcontent(contents) or obj.addcontent(box, contents).

src/nhlobj.c

@@ -106,7 +106,7 @@ l_obj_getcontents(lua_State *L)
 
 /* Puts object inside another object. */
 /* local box = obj.new("large chest");
-   box.addcontent(obj.new("rock"));
+   box:addcontent(obj.new("rock"));
 */
 static int
 l_obj_add_to_container(lua_State *L)