Improves clarity in Sphinx documentation for function signature.

[arthuRHD]

closes #11695.

[nicoddemus]

Thanks @arthuRHD!

[bluetech]

To be precise, None does not mean the same as 0:

• Setting returncode=0 means "exit with 0" (i.e. override the exit code to 0).
• Setting returncode=None means "exit with session.exitstatus" (i.e. don't override). Normally session.exitstatus is 0 but it can be set to something else by plugins. Even though session.exitstatus is undocumented some plugins do set it.

[arthuRHD]

Thanks @arthuRHD!

You guys did everything, thanks