Fix filter_input() return value documentation (#5381)

filter_input() returns null (not false) when the variable is not set.
Also clarify the FILTER_NULL_ON_FAILURE behavior which inverts the
return values: false when not set, null on filter failure.

Fixes #4361

Author: lacatoire

reference/filter/functions/filter-input.xml

@@ -57,11 +57,12 @@
  <refsect1 role="returnvalues">
   &reftitle.returnvalues;
   <simpara>
-   On success returns the filtered variable.
-   If the variable is not set &false; is returned.
-   On failure &false; is returned,
-   unless the <constant>FILTER_NULL_ON_FAILURE</constant> flag is used,
-   in which case &null; is returned.
+   Value of the requested variable on success,
+   &false; if the filter fails,
+   or &null; if the <parameter>var_name</parameter> variable is not set.
+   If the flag <constant>FILTER_NULL_ON_FAILURE</constant> is used,
+   it returns &false; if the variable is not set
+   and &null; if the filter fails.
   </simpara>
  </refsect1>