Clarify status of specification and informal feature specifications (#6554)

Resolves #6088

Author: parlough

src/content/resources/language/spec/index.md

@@ -10,7 +10,8 @@ For a gentler introduction to Dart, see the
 
 ## Dart 3
 
-The Dart 3 language specification is in progress.
+The Dart 3 language specification is in progress and
+not yet up to date for the current release and language features.
 You can find the in-progress specification in PDF format:
 
 * [Latest, in-progress specification][latest draft]
@@ -25,6 +26,9 @@ informal language feature specifications in the [dart-lang/language][] repo:
 * [Accepted informal proposals][]
 * [Drafts of potential features][]
 
+Over time, these feature specifications will be incorporated into
+the official specification.
+
 [dart-lang/language]: {{site.repo.dart.lang}}
 [Accepted informal proposals]: {{site.repo.dart.lang}}/tree/main/accepted
 [Drafts of potential features]: {{site.repo.dart.lang}}/tree/main/working
@@ -42,8 +46,8 @@ The Dart 2 language specification is available in PDF format:
 
 [2-10 formal spec]: /resources/language/spec/versions/DartLangSpec-v2.10.pdf
 
-For information on Dart versions 2.12 and later, 
-which have support for [null safety](/null-safety), 
+For information on Dart versions 2.12 and later,
+which have support for [null safety](/null-safety),
 check out the in-progress [Dart 3 specification](#dart-3).
 
 ## Dart 1.x