conf: Document variables and config files/dirs

conf/strongswan.conf.5.tail.in

@@ -435,10 +435,30 @@ Retransmission	Relative Timeout	Absolute Timeout
 5	42s	89s
 giving up	76s	165s
 .TE
+.
+.SH VARIABLES
+.
+The variables used above are configured as follows:
 
+.nf
+.na
+${piddir}               @piddir@
+${prefix}               @prefix@
+${random_device}        @random_device@
+${urandom_device}       @urandom_device@
+.ad
+.fi
+.
 .SH FILES
-/etc/strongswan.conf
-
+.
+.nf
+.na
+/etc/strongswan.conf       configuration file
+/etc/strongswan.d/         directory containing included config snippets
+/etc/strongswan.d/charon/  plugin specific config snippets
+.ad
+.fi
+.
 .SH SEE ALSO
 \fBipsec.conf\fR(5), \fBipsec.secrets\fR(5), \fBipsec\fR(8), \fBcharon-cmd\fR(8)