docs: Add 'uid' and 'fid' information
docs: Add 'uid' and 'fid' information

Update 'Device address' section to describe 'zpci' element and
its two attributes 'uid' and 'fid'.

Signed-off-by: Yi Min Zhao <5bdf0032410856ea949e1abce934f6d55d9c7ba6@linux.ibm.com>
Reviewed-by: Boris Fiuczynski <2648abdd55bfebf313fd324b464b047e6eea300e@linux.ibm.com>
Reviewed-by: Stefan Zimmermann <3585c61fa02b3a7864ea2efc0bffd1e26decea6c@linux.ibm.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

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