Update documentation

[oboroc]

Hi @Fubukimaru,

I'd like to see a few tweaks to documentation:

• small formatting and wording tweaks;
• add new command-line parameter description;
• add Ada-style '--' description;
• add predefined labels, as per santiontanon request here: https://www.msx.org/forum/msx-talk/development/how-can-i-compile-asmsx-in-windows-10?page=1#comment-394734

[Fubukimaru]

Thanks!