Indicate which methods are disabled experimental features in the main zcash-cli help output.

[nathan-at-least]

When I run zcash-cli help I get a list of many methods. Then if I run zcash-cli help $SOMEMETHOD sometimes that help text says the feature is a disabled experimental feature.

It would be nice to segregate the disabled features in the top-level help output, so that it ends with a separate section that looks something like this:

== Experimental Features ==
**Note:** Experimental Features are DISABLED

**Note:** Experimental Features provide new functionality which has received the standard review and testing of other `zcashd` features. However, they typically have not had widespread use in production and their behavior or interface may change between releases. We do not recommend using them in large scale production environments. To enable them pass `zcashd` the `-experimentalfeatures` commandline option or add `experimentalfeatures=1` to `zcash.conf`.

z_mergetoaddress ["fromaddress", ... ] "toaddress" ( fee ) ( transparent_limit ) ( shielded_limit ) ( memo )
z_fancynewthingy arg1 ( arg2 )
…

If they are enabled, the methods would appear in the appropriate section.

Another approach would be to mark them "inline" somehow whether or not they are enabled or disabled, but that seems messier to me.