lpr: add more accurate comments

*: trailing whitespace removal

printutils/lpd.c

@@ -34,6 +34,9 @@
  * where XXX is the job number and HHHH is the client hostname. Unless a printing helper application
  * is specified lpd is done at this point.
  *
+ * NB: file names are produced by peer! They actually may be anything at all!
+ * lpd only sanitizes them (by removing most non-alphanumerics).
+ *
  * If HELPER-PROG (with optional arguments) is specified then lpd continues to process client data:
  * 	1. it reads and parses control file (cfA...). The parse process results in setting environment
  * 	variables whose values were passed in control file; when parsing is complete, lpd deletes
@@ -50,12 +53,16 @@
  * $M = the user to whom a mail should be sent if a problem occurs
  * $l = name of datafile ("dfAxxx") - file whose content are to be printed
  *
+ * lpd also provides $DATAFILE environment variable - the ACTUAL name
+ * of the datafile under which it was saved.
+ * $l is not reliable (you are at mercy of remote peer), DON'T USE IT.
+ *
  * Thus, a typical helper can be something like this:
  * #!/bin/sh
  * cat "$l" >/dev/lp0
  * mv -f "$l" save/
- * 
  */
+
 #include "libbb.h"
 
 // strip argument of bad chars