To provide help for users of your plugin, you should ship with a README.html file.
This file should be a standard HTML file, so it is viewable with any browser. Additionally, you should attach HTML IDs to paragraphs or DIVs which describe your settings.
The format of the IDs is:
So, if you have a setting "foobar" in your plugin "myplugin_plugin" you should use
id="myplugin_plugin_foobar" with the paragraph that gives more details on your setting and b2evolution will create a help icon that links to this paragraph next to the setting on the "Edit plugin settings" page.
You should also use phpdoc comments in your PHP code, to describe the file, class, members and methods.
Please refer to https://phpdoc.org/docs/latest/index.html for details.