Parenthesis are mandatory with rvalue-functions

This fixes a problem with the reference documentation where it was not
explained that parenthesis are only optional for statement-level
functions.

Here is an example :

    $a = function "argument" $b = literal

With the "old" parser, this will result in a parse error. With the new
parser, it seems that it is interpreted like this :

    $a = 'function';
    'argument';
    $b = 'literal';

Which might cause puzzling bugs.

source/puppet/3/reference/lang_functions.markdown

@@ -44,7 +44,7 @@ In the examples above, `template`, `include`, and `str2bool` are all functions.
 The general form of a function call is:
 
 * The name of the function, as a bare word
-* An optional opening parenthesis
+* An opening parenthesis, that is optional when the function is used as a statement
 * Any number of **arguments,** separated with commas; the number and type of arguments are controlled by the function
 * A closing parenthesis, if an open parenthesis was used