Add comments regarding temp dir usage/setting.

ctrl/sbbs.ini

@@ -16,6 +16,9 @@
 ; Override the SBBSCTRL environment variable (optional):
 	CtrlDirectory=
 ; Override the default system "temp" directory (optional):
+; If you do override this, do NOT use the system-wide temp dir (ie: /tmp or
+;   windows\temp... use a subdirectory.  Synchronet assumes it is the only user
+;   of this temp dir.
 	TempDirectory=
 ; Increase value to decrease semaphore file checking frequency:
 	SemFileCheckFrequency=5	; seconds