Documentation: PCI: endpoint: Clarify pci_epc_set_bar() usage

Documentation: PCI: endpoint: Clarify pci_epc_set_bar() usage

The current documentation implies that pci_epc_set_bar() is only used
before the host enumerates the endpoint.

In practice, some Endpoint Controllers support calling pci_epc_set_bar()
multiple times for the same BAR (without clearing it) in order to update
inbound address translations after the host has programmed the BAR base
address, which some Endpoint Functions such as vNTB already rely on.
Add document text for that.

Also document the expected call flow for BAR subrange mapping
(pci_epf_bar.num_submap / pci_epf_bar.submap), which may require a
second pci_epc_set_bar() call after the host has programmed the BAR base
address.

Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260124145012.2794108-6-den@valinux.co.jp