Updates following review

* Added missing "discoverable" for node update transaction message name.
* Added missing detail in the "Discoverable Node" definition.
* Clarified that account ID is not relevant to discoverable nodes.
* Added a specification for the value in state used to store the
  discoverable node information.
* Added detail for admin key usage, network throttles, and related
  information to the security implications section.
* Added an SDK Considerations section.
* Added Mirror Node as a node type.
* Cleaned up some specification wording.
* Added some detail to the "reputation" open issue.
* Added some initial use cases, community ideas and revision are encouraged.

Signed-off-by: Joseph Sinclair <121976561+jsync-swirlds@users.noreply.github.com>

Author: jsync-swirlds