Fix incorrect async/await behavior explanation in sample comments (fixes #421)
Remove undesired reference to synchronous extensions documentation
Add external link for async/await overview
Clean up explanation of the single-layer AggregateException guarantee
Json.NET 5.0.8 is unable to (directly) deserialize a null JToken value in the extension dictionary (fixes #419)
Merge pull request #423 from sharwell/fix-419

Json.NET 5 can't handle null values in extension data
Merge pull request #424 from sharwell/fix-421

Fix 421