Clarify use of textual timestamps for event ops

I agree with the message here, but feel it should state **should** vs **must**. Technically, you do not need to use a millis format for targeting IF the event you are targeting has 0 for the millis portion. When using a non-millis format, the millis portion just defaults to 0. For example, I could POST to 
```
"https://api.orchestrate.io/v0/jimbotest/foo/events/test/19941106T084937Z" --data '{"foo":"Bar"}'
```
then get a header back that is
```
Location: /v0/jimbotest/foo/events/test/784111777000/10001
```
And the fetch with the same non-millis string format (instead of the long millis form) via a GET:
```
"https://api.orchestrate.io/v0/jimbotest/foo/events/test/19941106T084937Z/10001"
```