bpo-16500: Don't use string constants for os.register_at_fork() behavior (#1834)

Instead use keyword only arguments to os.register_at_fork for each of the scenarios.
Updates the documentation for clarity.
diff --git a/Doc/c-api/sys.rst b/Doc/c-api/sys.rst
index c6777d6..95d9d65 100644
--- a/Doc/c-api/sys.rst
+++ b/Doc/c-api/sys.rst
@@ -49,9 +49,10 @@
 
 .. c:function:: void PyOS_AfterFork_Child()
 
-   Function to update some internal state after a process fork.  This
-   should be called from the child process after calling :c:func:`fork`
-   or any similar function that clones the current process.
+   Function to update internal interpreter state after a process fork.
+   This must be called from the child process after calling :c:func:`fork`,
+   or any similar function that clones the current process, if there is
+   any chance the process will call back into the Python interpreter.
    Only available on systems where :c:func:`fork` is defined.
 
    .. versionadded:: 3.7