The -- to the long – is a conversion as explained in:
https://www.dokuwiki.org/wiki:syntax#text_to_html_conversions
You can prevent interpretation as syntax by wrapping with %%..%% for example in your case: man %%--%%help or %%man --help%%
or placing it in code blocks. Here 3 examples: <code>man --help</code>, <file>man --help</file> or prefixing by two spaces " man --help"