Reduce specific guidance in textual formats principle and link to other principles instead

[LeaVerou]

We just merged PR #505 but a lot of its guidance needs to be split out to separate principles, e.g.:

• should we have a principle about syntactic flexibility?
• should we have a principle about specifications needing to fully define processing and error handling so that all inputs result in consistent outcomes?

Then this principle should link to those and any other relevant principles (PoC, consistency, tradeoffs between high-level and low-level APIs, making simple things easy and complex things possible, etc.)

[martinthomson]

The text in that pull request cited https://w3ctag.github.io/design-principles/#avoid-ambiguity for the latter one. I think that we can do more on the former. There are plenty of interesting factors to balance.