add nodes for sectioning commands in the default case for plaintext output

* doc/texinfo.texi (Global Customization Variables), tta/C/texi2any.c
(formats_table), tta/perl/texi2any.pl (%formats_table): set
auto_section_node and no_warn_duplicate_section_target in formats
table for plaintext.

* tta/perl/t/plaintext_tests.t (no_node_with_section_split),
tta/perl/t/test_dirs_generated_list.txt,
tta/perl/t/test_files_generated_list.txt: add tests of plaintext split
by node with sectioning commands without nodes.