docs(core): EventEmitter docs for isAsync defaults

[dherges]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

Solves #15758

What is the new behavior?

Solves #15758

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

Other information:

[dherges]

@IgorMinar @mhevery @tbosch @vicb Getting feedback (whether positive or negative) would be nice!

[tbosch]

Looks good, please update the commit message to docs(core): ...

[dherges]

@tbosch Done. Thanks for your reply!

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