Add examples for filter_var_array() and filter_input_array()

[lacatoire]

Fixes #4218

Add examples for filter_var_array() and filter_input_array(), and correct the descriptions that those examples contradicted.

filter_var_array() gains two examples next to the existing one: applying a single filter to every entry of the array, and FILTER_CALLBACK. filter_input_array() had no examples at all and gains three: filtering GET input, filtering POST input (including a field that was not submitted, to show what add_empty does), and requesting an input type that is not populated.

Writing those examples surfaced several inaccuracies in the surrounding prose, corrected here:

• filter_input_array() returns null, not false, when the input array designated by type is not populated. This does not depend on any flag.
• A missing entry is always added as null when add_empty is true. The FILTER_NULL_ON_FAILURE flag does not turn it into false; that inversion applies to filter_input(), not to the array form.
• filter_var_array() had no mention of add_empty or FILTER_NULL_ON_FAILURE in its return values, and stated that a failing entry is false without either qualifier.
• With FILTER_FORCE_ARRAY, the failure value is wrapped in a one element array like any other result.
• The options description is shared with filter_input_array() through xi:include, but referred to $array, a parameter that only exists on filter_var_array().
• FILTER_REQUIRE_SCALAR is the default for both functions, so the flag in the first example only restates it. Said so, rather than leaving it to look like it changes the outcome.
• Typo in the shared options description: "specifies and flags".

All outputs were reproduced on PHP 8.4.24, the request-dependent ones through the built-in web server.

Sources

• null returned whenever the input array is absent, independently of any flag: ext/filter/filter.c, PHP_FUNCTION(filter_input_array), if (!array_input) { RETURN_NULL(); }
• Missing keys always added as null: ext/filter/filter.c, php_filter_array_handler(), add_assoc_null_ex() is reached without the flags being read
• FILTER_REQUIRE_SCALAR reinstated as the default whenever no *_ARRAY flag is given: ext/filter/filter.c, php_filter_call()