DOC: internals: Update the HTX API documentation for tunneled data

DOC: internals: Update the HTX API documentation for tunneled data

The HTX API documentation was updated to reflect the addition of the RAW_DATA
block type:

  * The new block type was added in the list of supported types and in the
    typical composition of an HTX message, the tunneled data being always
    located after the end of the message.

  * The section about the end of the message was rewritten. Tunneled data are
    no longer stored in a separate buffer, they have their own block type. It
    is now stated that testing the tail block is enough because a RAW_DATA
    block may only be found after the end of the message. It is also stated
    that the block carrying the flag may already have been consumed, an HTX
    message being a partial and stateless representation of an HTTP message.

  * htx_add_raw_data(), htx_add_data_type(), htx_add_data_type_atonce(),
    htx_is_data_type() and htx_msg_ended() were documented, the last one
    replacing htx_has_eom(). It is also stated that htx_add_last_data() is
    dedicated to regular DATA blocks and that htx_defrag() merges consecutive
    blocks of the same payload type.