`mb_strpos`: add example and clarify return value section

[homersimpsons]

Affected page

https://www.php.net/manual/en/function.mb-strpos.php

Current issue

It is unclear from the documentation whether mb_strpos returns a byte offset or a char offset. It returns a char offset:

var_dump(mb_strpos("🐘H🐘é", "é")); // => 3
var_dump(mb_strpos("🐘H🐘é", "🐘", 1)); // => 2

Suggested improvement

An example can be enough, wording can be improved too as "numeric position" does not indicate how is this position interpreted (in my opinion). I cannot think of a great wording yet.

Additional context (optional)

No response