Explain how uidNumber are created

Based on the code uidNumber must be created at these lines

https://gitlab.fusiondirectory.org/fusiondirectory/fd-plugins/-/blob/1.4-dev/posix/personal/posix/class_posixAccount.inc#L913 https://gitlab.fusiondirectory.org/fusiondirectory/fd-plugins/-/blob/1.4-dev/posix/personal/posix/class_posixAccount.inc#L951