Fix formatting in release-process.md
Update Gitian Build guide to include OSX
Clarify error message when invalid -rpcallowip

Also add to HelpMessage() what specifications are valid.
Merge pull request #4454

9d5ad71 Fix formatting in release-process.md (Michael Ford)
Merge pull request #4455

3f7a61f Update Gitian Build guide to include OSX (Michael Ford)