net: pse-pd: tps23881: Clarify setup_pi_matrix callback documentation

net: pse-pd: tps23881: Clarify setup_pi_matrix callback documentation

Improve the setup_pi_matrix callback documentation to clarify its purpose
and usage. The enhanced description explains that PSE PI devicetree nodes
are pre-parsed before this callback is invoked, and drivers should utilize
pcdev->pi[x]->pairset[y].np to map PSE controller hardware ports to their
corresponding Power Interfaces.

This clarification helps driver implementers understand the callback's
role in establishing the hardware-to-PI relationship mapping.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20250620-poe_doc_improve-v1-2-96357bb95d52@bootlin.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>