Decide and document allowed format for property names

I don't think we currently specify anywhere how property names should look (what characters they may contain, etc.). For example, colons are definitely forbidden, but whether everything else is allowed is not clear.
We might also want to reserve one or two more characters for future use. And it's also a question of whether Core's Typed Data API can deal with those characters.
Maybe we should also clearly state that property names defined by processors should have the module identifier as prefix, to avoid clashes. (The Search API itself used to do this in D7, too, but doesn't anymore – but clashes with our property names should be readily visible anyways.)

(For datasource IDs, see #2565045: Document valid plugin ID format in Core.)

Estimated Value and Story Points

This issue was identified as a Beta Blocker for Drupal 8. We sat down and figured out the value proposition and amount of work (story points) for this issue.

Value and Story points are in the scale of fibonacci. Our minimum is 1, our maximum is 21. The higher, the more value or work a certain issue has.

Value : 1
Story Points: 1