[7.1.0] Make some minor adjustments to the compact execution log format and document it better. (#21146)

* Remove the Output top-level payload type. It can only appear as a
sub-message of a Spawn.
* Add an Invocation top-level payload type. It is currently empty but
reserved for anticipated needs (e.g. to report the digest function
used).
* Spell out some guarantees (and non-guarantees) about the format.

Renumbering is fine, since the format hasn't made it into a release yet.

PiperOrigin-RevId: 602352295
Change-Id: I6044c9a9a51360dfce5701071883b318b1e9a5c2