>WIth the help output that they provide, how am I supposed to know that
os.fork is only supported on Unix.

On windows, os.fork does not exist, so help(os.fork) fails, so it is obvious to me that it is not available on Windows ;-).

help(winreg) returns
"help(winreg)
Help on built-in module winreg:

NAME
    winreg - This module provides access to the Windows registry API."

One could, for example, suggest "add 'Windows-only' between 'This' and 'module' in the module doc string that follows the name." This is not needed here because 'Windows' appears later. 

However, if you want to move this issue forward, use the quoted example as a model for a list of specific suggestions for objects specific to platforms you have access to. (For instance, I cannot easily get the os.fork doc string to see what it now says.) User suggestions are the main way existing docs get improved.

In other words, check the existing doc string and see if it is deficient. If it is, quote is and suggest a change. If the suggested change is the same for several objects, list and quote several and then make the suggestion, such as "insert 'unix-only' before 'module'."