Is your enhancement proposal related to a problem? Please describe.
With #27901 and #29074 it has become clear that current documentation is insufficient in its description on the processing order of shields and boards for overlay files.

Describe the solution you'd like
A clear description in the user guide https://docs.zephyrproject.org/latest/guides/porting/shields.html on processing order of shields and boards, and why this order is important / has been decided.

Look at:
#29074 (comment)
for inspiration.