NFS: Update idmapper documentation

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Id mapper is used by NFS to translate user and group ids into names, and to

translate user and group names into ids.  Part of this translation involves

performing an upcall to userspace to request the information.  Id mapper will

user request-key to perform this upcall and cache the result.  The program

/usr/sbin/nfs.idmap should be called by request-key, and will perform the

translation and initialize a key with the resulting information.

performing an upcall to userspace to request the information.  There are two

ways NFS could obtain this information: placing a call to /sbin/request-key

or by placing a call to the rpc.idmap daemon.

NFS will attempt to call /sbin/request-key first.  If this succeeds, the

result will be cached using the generic request-key cache.  This call should

only fail if /etc/request-key.conf is not configured for the id_resolver key

type, see the "Configuring" section below if you wish to use the request-key

method.

If the call to /sbin/request-key fails (if /etc/request-key.conf is not

configured with the id_resolver key type), then the idmapper will ask the

legacy rpc.idmap daemon for the id mapping.  This result will be stored

in a custom NFS idmap cache.

 NFS_USE_NEW_IDMAPPER must be selected when configuring the kernel to use this

 feature.

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Configuring