ICS20-1 cleanup

[ethanfrey]

When preparing to add ICS20-2 documentation, I realized the spec deviated from the implementation, and a few items were underspecified.

In particular:

• There is no source field in the packet, but important info is encoded in denomination, which is not clearly specified in human words (just pseudocode).
• The acknowledgement type was incorrect (it is enum with strings, not always success: boolean)
• It is not defined that the packet data and acknowledgement are JSON-encoded (at least one independent implementation used Protobuf encoding until they debugged compatibility issues)
• The prefix in the pseudo-code is missing a trailing / that is present in the implementation (and correct in the implementation I believe)

I added these suggests on a "best effort" basis and if the core devs disagree with any of the description, I would ask them to kindly correct this PR. In any case, this PR should be "more correct" than the current master.

[AdityaSripal]

ACK, pending @colin-axner 's clarification.

Thanks for the fix @ethanfrey

[cwgoes]

ACK but see comments - thanks for the improvements!

[cwgoes]

@milosevic Want to take a quick look at this or shall we merge (already 2/3 approvals)?

[milosevic]

Really nice work.

[cwgoes]

Merged with 3/3. Thank you @ethanfrey!