Undocument nmap.sleep and undeprecate stdnse.sleep.

There are various functions in the internal nmap.socket and nmap.dnet
libraries that are there for technical reasons:
	http://seclists.org/nmap-dev/2012/q1/318
The sleep function is exposed through the stdnse namespace because it
fits that library better.

Some code comments said that names such as nmap.new_socket were
deprecated in favor of e.g. nmap.socket.new, but the old names were
never formally deprecated, so I removed those comments.

nselib/stdnse.lua

@@ -41,10 +41,11 @@ local EMPTY = {}; -- Empty constant table
 
 _ENV = require "strict" {};
 
---- (Deprecated Alias) Sleeps for a given amount of time.
---
--- Please use nmap.sleep instead.
+--- Sleeps for a given amount of time.
 --
+-- This causes the program to yield control and not regain it until the time
+-- period has elapsed. The time may have a fractional part. Internally, the
+-- timer provides millisecond resolution.
 -- @name sleep
 -- @class function
 -- @param t Time to sleep, in seconds.