Documentation: Update filesystems/debugfs.txt

This patch update the Documentation/filesystems/debugfs.txt
file. The main work is to add the description of the following
functions:
    debugfs_create_atomic_t
    debugfs_create_u32_array
    debugfs_create_devm_seqfile
    debugfs_create_file_size

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>