Documented attempt to make code thread and asynchronous I/O safe

Added the sentence "While every effort has been made to make the code thread and
asynchronous I/O safe, it is always best to have each process write to
its own dedicated logger file."

[ticket: X]

Author: wclodius2

doc/specs/stdlib_logger.md

@@ -34,6 +34,10 @@ The logger variables have the option to:
 * indent subsequent lines of the messages; and
 * format the text to fit within a maximum column width.
 
+While every effort has been made to make the code thread and
+asynchronous I/O safe, it is always best to have each process write to
+its own dedicated logger file.
+
 Note: Loggers of type `logger_type` normally report their messages to I/O
 units in the internal list termed `log_units`. However if `log_units`
 is empty then the messages go to the `output_unit` of the intrinsic