More actionable error message when spawn is incorrectly used. (#102203)

Co-authored-by: Yuxin Wu <[email protected]>
Co-authored-by: Oleg Iarygin <[email protected]>

Author: ppwwyyxx

Lib/multiprocessing/spawn.py

@@ -150,7 +150,11 @@ def _check_not_importing_main():
                 ...
 
         The "freeze_support()" line can be omitted if the program
-        is not going to be frozen to produce an executable.''')
+        is not going to be frozen to produce an executable.
+
+        To fix this issue, refer to the "Safe importing of main module"
+        section in https://docs.python.org/3/library/multiprocessing.html
+        ''')
 
 
 def get_preparation_data(name):