add passphrase info to http-server.1 in doc

Author: chris--jones

doc/http-server.1

@@ -110,6 +110,7 @@ If not specified, uses cert.pem.
 .BI \-K ", " \-\-key " " [\fIFILE\fR]
 Path to SSL key file.
 If not specified, uses key.pem.
+Passphrase will be read from NODE_HTTP_SERVER_SSL_PASSPHRASE (if set)
 
 .TP
 .BI \-r ", " \-\-robots " " [\fIUSER\-AGENT\fR]