Correct inline docs for `is_wp_error()`

[johnbillion]

The inline docs for is_wp_error() states:

Does not check to see if the parent is also WP_Error, so can't inherit WP_Error and still use this function.

This is incorrect. is_wp_error() will return the expected result when passed an object of a class which extends WP_Error because it uses is_a() internally. From the ​`is_a()` man page:

Checks if the object is of this class or has this class as one of its parents.

Introduced in r8900.

[SergeyBiryukov]

In 24507:

Correct inline docs for is_wp_error(). props johnbillion. fixes #24637.