We need this to handle equally named properties in different
modules.
* src/tools/docmaker/content.py (re_identifier),
src/tools/docmaker/sources.py (re_crossref): Allow `foo[bar]'.
* src/tools/docmaker/tohtml.py (HtmlFormatter::make_html_word,
HtmlFormatter::index_exit, HtmlFormatter::section_enter,
HtmlFormatter::block_enter): Handle `foo[bar]'.