Update comment note for from-pstn-custom

In the file extensions.conf:

;-------------------------------------------------------------------------------
; from-pstn:
;
; Entry context for calls from the outside world to hit FreePBX
[from-pstn]
include => from-pstn-custom             ; create this context in extensions_custom.conf to include customizations

Every line that's added to the context from-pstn-custom, will supersede a line that's generated by FreePBX and some of those early lines are important for security. Ultimately, we should deprecate and remove support for the from-pstn-custom context, but until then, lets update the comment to read:

; this context include remains for legacy reasons, but should not be used.