Some users seem to not know about how `PATH` lookup works. I haven't gone back and found all the related issues, but I remember discussing/explaining it numerous times as part of what seem like unrelated issues. I also remember numerous occasions where users had problems where part of the troubleshooting has been to check whether the build command works on its own, outside of Geany, which is effectively the same topic.

I'm creating this issue as a reminder to revise the documentation to go into more detail on how this mechanism works and how to configure it on various platforms.

Also related, but perhaps deserving its own separate issue, I think it might be useful to incorporate @elextr's [Build Menu Tutorial](https://wiki.geany.org/howtos/configurebuildmenu) directly into the main user manual as well.