dynamic_debug documentation: minor fixes

For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is

its ``prefix_str`` argument, if it is constant string; or ``hexdump``

in case ``prefix_str`` is build dynamically.

in case ``prefix_str`` is built dynamically.

Dynamic debug has even more useful features:

    line number matches the callsite line number exactly.  A

    range of line numbers matches any callsite between the first

    and last line number inclusive.  An empty first number means

    the first line in the file, an empty line number means the

    last number in the file.  Examples::

    the first line in the file, an empty last line number means the

    last line number in the file.  Examples::

	line 1603           // exactly line 1603

	line 1600-1605      // the six lines from line 1600 to line 1605