Improve inline docs description for WP_Error::__construct()

[jdgrimes]

The docblock description of WP_Error::__construct() is confusing.

         * If code parameter is empty then nothing will be done. It is possible to
         * add multiple messages to the same code, but with other methods in the
         * class.
         *
         * All parameters are optional, but if the code parameter is set, then the
         * data parameter is optional.

Introduced in [8900].

[jdgrimes]

First pass, could probably be much improved

[DrewAPicture]

I agree the original description is confusing. It would be nice to make the second part less ambiguous. "other methods" is pretty vague:

It is possible to add multiple messages to the same code by using other methods in the class.

Can we be more specific here?

[jdgrimes]

Be less ambiguous, improve add() docs as well

[jdgrimes]

28203.2.patch​ removes the ambiguity, and improves the summary for the add() method as well.

[DrewAPicture]

28203.2.patch​ is much better, thanks.

[DrewAPicture]

In 28660:

Improve inline documentation for the __construct() and add() methods in WP_Error class.

Props jdgrimes.
Fixes #28203.