Documentation: boot.rst: fix warnings

  chunks of memory are occupied by kernel data.

  Thus setup_indirect struct and SETUP_INDIRECT type were introduced in

  protocol 2.15.

  protocol 2.15::

    struct setup_indirect {

      __u32 type;

  and stack space can be limited in boot contexts.

  Let's give an example how to point to SETUP_E820_EXT data using setup_indirect.

  In this case setup_data and setup_indirect will look like this:

  In this case setup_data and setup_indirect will look like this::

    struct setup_data {

      __u64 next = 0 or <addr_of_next_setup_data_struct>;

All kernel_info data should be part of this structure. Fixed size data have to

be put before kernel_info_var_len_data label. Variable size data have to be put

after kernel_info_var_len_data label. Each chunk of variable size data has to

be prefixed with header/magic and its size, e.g.:

be prefixed with header/magic and its size, e.g.::

  kernel_info:

          .ascii  "LToP"          /* Header, Linux top (structure). */