Documented the SNAC_BASEDIR environment variable.

doc/snac.1

@@ -377,6 +377,13 @@ https://$SNAC_HOST/oauth/x-snac-get-token
 .Pp
 .Sh ENVIRONMENT
 .Bl -tag -width Ds
+.It SNAC_BASEDIR
+This optional environment variable can be set to the base directory of
+your installation; if set, you don't have to add the base directory as an
+argument to command-line operations. This may prove useful if you only
+have one
+.Nm
+instance in you system (which is probably your case).
 .It Ev DEBUG
 Overrides the debugging level from the server 'dbglevel' configuration
 variable. Set it to an integer value. The higher, the deeper in meaningless