DocMaker Help: Mention Linux & macOS

[tajmone]

Currently the PureBasic DocMaker Help documentation (now converted to AsciiDoc) only mentions the DocMaker.exe binary found in the PB SDK for Windows:

• DocMaker-Help.html

The document should be updated to include info for the other supported OSs too:

• [ ] Mention DocMaker for macOS (binary, its location in SDK, and other useful info).
• [ ] Mention DocMaker for Linux (binary, its location in SDK, and other useful info).

Also, in §3. Command Line Usage, for the /OS parameter it states:

/OS The OS for which the documentation will be compiled. It can be either Windows, Linux, or AmigaOS.

For some reason, macOS is not mentioned, whereas Amiga is still mentioned. Since there's no command line help option we have no way of checking whether some parameters have changed since the DocMaker Help was last updated:

• [ ] @fantaisie-software, could you pleas confirm for the /OS parameter:
  • [ ] Is there an option for macOS?
  • [ ] Is AmigaOS still supported in DocMaker v5.50?
    • [ ] Amend document accordingly...