Documentation: Exporting: update description of d_splice_alias

Following commits a9049376 and 0c1aa9a9 update the d_splice_alias
desciption.
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/nfs/Exporting

@@ -92,7 +92,14 @@ For a filesystem to be exportable it must:
    1/ provide the filehandle fragment routines described below.
    2/ make sure that d_splice_alias is used rather than d_add
       when ->lookup finds an inode for a given parent and name.
-      Typically the ->lookup routine will end with a:
+
+      If inode is NULL, d_splice_alias(inode, dentry) is eqivalent to
+
+		d_add(dentry, inode), NULL
+
+      Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err)
+
+      Typically the ->lookup routine will simply end with a:
 
 		return d_splice_alias(inode, dentry);
 	}