linter: Add documentation to the 'unnecessary_unawaited' lint rule

Change-Id: I2e46a3bdbbe40ae3e0e383eb75357ea38ea95596
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/428581
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

https://dart.googlesource.com/sdk/+/20755e2eb7acd3190eeb6c6e010a6cc39ec0ddd9
diff --git a/DEPS b/DEPS
index f50c2dc..f93ff66 100644
--- a/DEPS
+++ b/DEPS
@@ -234,7 +234,7 @@
 ]
 
 deps = {
-  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'd5b82a3ecdd6c81d82f46c214f68f650bfdcc05a',
+  'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '20755e2eb7acd3190eeb6c6e010a6cc39ec0ddd9',
 
   'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '146a338036e8fdcbfaa764ca458fff2073c94379',
 
diff --git a/commits.json b/commits.json
index 724e635..2e9dd00 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
   "flutter":"146a338036e8fdcbfaa764ca458fff2073c94379",
-  "flutter/engine/src/flutter/third_party/dart":"d5b82a3ecdd6c81d82f46c214f68f650bfdcc05a"
+  "flutter/engine/src/flutter/third_party/dart":"20755e2eb7acd3190eeb6c6e010a6cc39ec0ddd9"
 }
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