man lsof: mention /etc/services for -P option (#309)

fixing issue #297

Author: wwwutz

Lsof.8

@@ -1401,7 +1401,7 @@ and take effect before other selection criteria are applied.
 .TP \w'names'u+4
 .B \-P
 inhibits the conversion of port numbers to port
-names for network files.
+names for network files (the mapping is commonly learned from /etc/services).
 Inhibiting the conversion may make
 .I lsof
 run a little faster.