DOC: Extensions options missing from sphinx-apidoc html docs

[rossbar]

Describe the bug
The --extensions flag and related --ext-* flags are not included in the docs for sphinx-apidoc [src] [rendered].

To Reproduce

Compare the output of sphinx-apidoc --help with the page linked above.

Environment info

• OS: Linux
• Python version: 3.9.1
• sphinx-apidoc --version = sphinx-apidoc 3.4.1