netfs: Documentation for helper library

Add interface documentation for the netfs helper library.

Signed-off-by: David Howells <dhowells@redhat.com>
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Link: https://lore.kernel.org/r/161539533275.286939.6246011228676840978.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653791767.2770958.2012814194145060913.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789072591.6155.9448294406920216219.stgit@warthog.procyon.org.uk/ # v6