This PR is an attempt to improve some of the Windows-related documentation as well as comments in some related files.

The main user-visible bit is a suggestion to change the man page which currently mentions:

-m syspath
       Set the dynamic binding :syspath to the string syspath  so  that
       Janet  will  load system modules from a directory different than
       the default. The default is set when Janet  is  built,  and
       defaults to /usr/local/lib/janet on Linux/Posix, and
       C:/Janet/Library on Windows. This option supersedes JANET_PATH.

AFAIU, unlike for non-Windows [1], on Windows, there is no default value baked in to janet.exe[2].

The proposed new text is:

-m syspath
       Set the dynamic binding :syspath to the string syspath so that
       Janet will load system modules from a directory different than
       the default. The default is set when Janet is built, and 
       defaults to /usr/local/lib/janet on Linux/Posix. On Windows,
       there is no default value. This option supersedes JANET_PATH.

Note that, if one installs from the official (or locally-built) .msi, "arrangements are made" (TM) so that JANET_PATH is set to some location under %APPDATA% [3]. Thus, subsequent typical invocations of janet.exe (e.g. those made via new command prompts) will have JANET_PATH set and consequently (dyn :syspath) will likely yield a non-nil string path.

While I was at it, I also tweaked the man page bit about JANET_PATH to have:

If no JANET_PATH is set, Janet will look in the default location set at compile time. This should be a colon-separated list of directory names on Linux/Posix, and a semicolon-separated list on Windows. Note that a typical setup (i.e. not NixOS / Guix) will only use a single directory.

[1] See here and here.

[2] This was also verified by manually building janet.exe from source on Windows and checking (dyn :syspath) (which returned nil) on a system that didn't have Janet installed via a typical .msi.

[3] A comment was added to tools/msi/janet.wxs to this effect.