Clarify how to document event details

[arthurevans]

Clarify doc on event details:

googlearchive/polymer-element-catalog#233

Add pseudoElement (for documenting non-element imports).

To help me prioritize and manage issues in the repo, I'll close this issue with no docs changes in 7 days. To prevent this, please comment within 7 days.

[arthurevans]

psuedoElement is no longer relevant in 2.x, I think, but improving the doc on event properties is still relevant.