Fix documentation for set-value Plugin

[jbigot]

As identified by @jmorice91 , can you explain what the issue is, Jacques?

[jmorice91]

suggest small change:

• Define the difference between share and expose with an example.
• In section "value_list", we have this remark:
  "The order of the elements is important. The descriptors will be set in the same order as given in the yaml file."
  I propose to add an example to show that and to refer to it in section "value_list".
• some typographical error to correct

[jbigot]

Thanks!