[php-src] Issue #15553: Stubs: internally modifyable properties are not @readonly

Issue: https://github.com/php/php-src/issues/15553 Author: cmb69 ### Description We have a couple of internal classes (e.g. mysqli) which implement properties basically as hooks (i.e. internally calling a function). Such properties should not be tagged as @readonly in the stubs, because their behavior is not what users expect (their value is not fixed), and it also can be very confusing regarding the behavior of ReflectionProperty::isReadOnly() et. al, see #15309. While at least for me this is more of a documentation issue, since the classsynopses are generated from the stub files, some improvements would need to be done there at first. ### PHP Version PHP 8 ### Operating System any