debugfs: add documentation for debugfs_create_x64

				      struct dentry *parent, u16 *value);

    struct dentry *debugfs_create_x32(const char *name, mode_t mode,

				      struct dentry *parent, u32 *value);

Note that there is no debugfs_create_x64().

    struct dentry *debugfs_create_x64(const char *name, mode_t mode,

				      struct dentry *parent, u64 *value);

These functions are useful as long as the developer knows the size of the

value to be exported.  Some types can have different widths on different