conf/rtl_433_example.conf is confusing (multiple things to fix)

[gdt]

(In #2100 someone had an issue with hopping being enabled when not expected.)

The config file does not have a clear strategy. One strategy (that I prefer) is to have all statements be 1) configured to produce the default behavior to the extent that is possible and 2) commented out, and to declare that.

The default config file has documentation about how config files are read. This is an example, and therefore not a correct place to document that -- that information should be in the man page.

There are flex decoders commented out. Presumably they should be in a file each, not commented out, so that they can be included if desired.