We need to document usage of '%' in PREFERRED_VERSION.
I can find basic usage of '%' in PREFERRED_VERSION in bitbake user manual https://www.yoctoproject.org/docs/2.4.1/bitbake-user-manual/bitbake-user-manual.html
This is already documented:https://www.yoctoproject.org/docs/2.4.1/bitbake-user-manual/bitbake-user-manual.html#var-PREFERRED_VERSION