Add aweful.util.pread() so API docs have deprication notice (#1502)

* Add awful.util.pread() so API docs have deprication notice

* Update util.lua

* Update util.lua

* Update util.lua

lib/awful/util.lua

@@ -33,6 +33,11 @@ util.table = {}
 --- The default shell used when spawing processes.
 util.shell = os.getenv("SHELL") or "/bin/sh"
 
+--- Execute a system command and road the output.
+-- This function implementation **has been removed** and no longer
+-- do anything. Use `awful.spawn.easy_async`.
+-- @deprecated awful.util.pread
+
 --- Display a deprecation notice, but only once per traceback.
 -- @deprecated deprecate
 -- @param[opt] see The message to a new method / function to use.