Fix SynExpr.ImplicitZero doc comment (#7831)

auduchinok · TIHan · commit 7a02f41fa19f · 2019-11-07T20:28:14.000-08:00
ImplicitZero is also used in error recovery during parsing, and the current comment contradicts it.
diff --git a/src/fsharp/ast.fs b/src/fsharp/ast.fs
@@ -748,7 +748,7 @@ and
     /// Computation expressions only, based on JOIN_IN token from lex filter
     | JoinIn of SynExpr * range * SynExpr * range: range
 
-    /// Used internally during type checking for translating computation expressions.
+    /// Used in parser error recovery and internally during type checking for translating computation expressions.
     | ImplicitZero of range: range
 
     /// Used internally during type checking for translating computation expressions.
