SUNRPC: xdr.h: fix all kernel-doc warnings

SUNRPC: xdr.h: fix all kernel-doc warnings

Correct a function parameter name (s/page/folio/) and add function
return value sections for multiple functions to eliminate
kernel-doc warnings:

Warning: include/linux/sunrpc/xdr.h:298 function parameter 'folio' not
 described in 'xdr_set_scratch_folio'
Warning: include/linux/sunrpc/xdr.h:337 No description found for return
 value of 'xdr_stream_remaining'
Warning: include/linux/sunrpc/xdr.h:357 No description found for return
 value of 'xdr_align_size'
Warning: include/linux/sunrpc/xdr.h:374 No description found for return
 value of 'xdr_pad_size'
Warning: include/linux/sunrpc/xdr.h:387 No description found for return
 value of 'xdr_stream_encode_item_present'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>