PCI: Document MPS parameters pci=pcie_bus_safe, pci=pcie_bus_perf, etc

Document PCIe bus MPS parameters pcie_bus_tune_off, pcie_bus_safe,
pcie_bus_peer2peer, pcie_bus_perf.

These parameters were introduced by Jon Mason <jdmason@kudzu.us> at commit
5f39e6705 and commit b03e7495a8.

[bhelgaas: mention hot-add for pcie_bus_peer2peer]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>