rewrite file's 'dest' argument

Previously, there were two entries for 'dest'. I've combined them in a
way that makes sense to me.

This will be superfluous when 'path' is made. Ideally, I'd like to see
'path' have the bulk of the definition, and 'dest' to be an alias to
'path'.

rst/modules.rst

@@ -185,6 +185,7 @@ All parameters available to the file module are also available when running the
 *dest*:
 
 * absolute path to a file on the filesystem.
+* alias for 'path'. Sets an absolute path to a file on the filesystem when used with 'state=file'. When used with 'state=link', sets the destination to create a symbolic link defined by 'src' key.
 
 *state*: