gog.exlib: improve docs

exlibs/gog.exlib

@@ -11,6 +11,8 @@
 #   exparams: - installer: the shell script installer (required)
 #             - unzip: whether to unzip and add unzip dependency (default: true)
 #             - install_dir: installation dir (default: /opt/${PN})
+#  variables: - GOG_SH: set to exparam installer
+#             - GOG_INSTALL_DIR: set to exparam install_dir, use this in src_install
 
 
 myexparam installer=