Update documentation for 'file' module to include 'diff_peek'.

The 'diff_peek' option isn't documented at all, and provides a
rudimentary check that the content isn't binary. Documentation is
added to explain the option.

The 'validate' option has a declaration, but isn't implemented.
Therefore it may as well be removed from the module.

lib/ansible/modules/files/file.py

@@ -87,6 +87,12 @@ options:
       - 'force the creation of the symlinks in two cases: the source file does 
         not exist (but will appear later); the destination exists and is a file (so, we need to unlink the
         "path" file and create symlink to the "src" file in place of it).'
+  diff_peek:
+    required: false
+    description:
+      - "Only check whether the file looks like binary. Returns with the parameter
+         'appears_binary' set to True or False depending on the initial content of the
+         file. This option is enabled when the option is set (to any value)."
 '''
 
 EXAMPLES = '''
@@ -158,7 +164,6 @@ def main():
             recurse  = dict(default='no', type='bool'),
             force = dict(required=False,default=False,type='bool'),
             diff_peek = dict(default=None),
-            validate = dict(required=False, default=None),
             src = dict(required=False, default=None),
         ),
         add_file_common_args=True,