Documentation/filesystems/seq_file.txt: create_proc_entry deprecated

Documentation/filesystems/seq_file.txt: create_proc_entry deprecated

Linked article in seq_file.txt still uses create_proc_entry which was
removed in commit 80e928f7ebb9 ("proc: Kill create_proc_entry()").

This patch adds information for kernel 3.10 and above

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>