Document each action

[abbyad]

In https://github.com/medic/medic-conf/pull/270 the actions listed in the README were cleaned up so that the formatting is a bit more consistent. It would be useful to app builders to have a place where all actions are listed out with descriptions on how to use them.

[SCdF]

I would ask that we extend the fn spec (from {requiredInstance, execute} so that they are available in code (probably as a short and long description). This allows us to automatically generate help more usefully and specifically at runtime:

> medic-conf --available-actions #or whatever it's called
...
<action-name>: <short description>
<action-name>: <short description>
...
> medic-conf <action-name> --help
<action-name>:
<long-description>
> 

etc