Clarify forward_static_call() documentation

[zonuexe]

The forward_static_call() and forward_static_call_array() descriptions implied that these functions simply call static methods.

This clarifies that they call a callback while preserving the current called class for method callbacks, making them useful for static method callbacks that should behave like forwarding calls such as parent:: or self::.

The forward_static_call_array() parameter note is also updated to match call_user_func_array(): parameters can be passed by reference when the corresponding element in the args array is a reference.

I am not a native English speaker, so I do not have strong preferences about the exact wording or nuance.

[lacatoire]

Thanks for the PR @zonuexe ! A commit landed on master (846dfb8) shortly after opening that covered the same ground. I'm closing to avoid conflict with it.

[zonuexe]

Rebased on latest master and resolved the conflict with 846dfb8. I kept that commit's correction that forward_static_call_array() does not require an active class scope, and adjusted this PR to focus on clarifying late static binding/current called class behavior and the args-by-reference note consistent with call_user_func_array().