Update comments to to_xfer_partial implementations.

Update comments to to_xfer_partial implementations.

Some comments to to_xfer_partial implementations are out of date.
This patch updates them using the "Implement the to_xfer_partial
target_ops method" pattern.

gdb:

2014-02-11  Yao Qi  <yao@codesourcery.com>

	* aix-thread.c (aix_thread_xfer_partial): Update comments.
	* auxv.c (procfs_xfer_auxv, memory_xfer_auxv): Likewise.
	* bsd-uthread.c (bsd_uthread_xfer_partial): Likewise.
	* gnu-nat.c (gnu_xfer_memory): Likewise.
	* inf-ptrace.c (inf_ptrace_xfer_partial):  Likewise.
	* rs6000-nat.c (rs6000_xfer_partial): Likewise.
	* sparc-nat.c (sparc_xfer_wcookie): Likewise.
	* spu-linux-nat.c (spu_proc_xfer_spu): Likewise.