docs: add more on transaction validity

[paulperegud]

We need more information on transaction validity for referencing from watcher-specs. Example questions implementer might have are:

• where to look for details?
• how does it work?

[pdobacz]

Nice! Posted some suggestions.

Can you double check that there isn't any description conflicting/duplicate with the one you've added? I vaguely recall reading something at least partially covering those topics.

Pls scan docs/ in general, if you haven't already.

[pdobacz]

its own set of rules

And then I'd add more explanation of the concept of a spending condition, before diving into links to code and "gory details" like the hash part. Like what is the abstraction, that a spending condition is defined per the pair tx_type output_type and so forth.

[paulperegud]

I've provided an example of how spending condition can be used, before the explanation of how it is implemented.

[pdobacz]

hm, the example is nice (consider choosing sth less speculative though, like a EC signature, which you can then link to our existing Payment tx spending condition). Also mind to keep it to a separate paragraph (check rendered .md)

However, I'd insist on providing some not-an-example explanation of a spending condition concept. Just explain what it is, in 2-3 sentences, one of them could literally be "a spending condition is defined per the pair of a transaction type and an output type".

[boolafish]

I vaguely recall reading something at least partially covering those topics.

Potentially this, though I think the duplication is quite small. But might have some place to optimize the links in the doc here: https://github.com/omisego/plasma-contracts/blob/master/plasma_framework/docs/design/payment-game-implementation-v1.md#transaction-data-structure

also, if fee is interested: https://github.com/omisego/plasma-contracts/blob/master/plasma_framework/docs/design/fee-game.md#fee-transaction-data-structure

[kevsul]

Looks good, some minor changes for readability

[pdobacz]

Just one outstanding comment (left a follow-up to one thread), then LGTM!