Clarify switch expression usage (dart-lang#4887)

Fixes dart-lang#4865 

---------

Co-authored-by: Anthony Sansone <[email protected]>
Co-authored-by: Parker Lougheed <[email protected]>

src/language/branches.md

@@ -129,20 +129,23 @@ check out the patterns documentation on [Switch statements and expressions][].
 
 ### Switch expressions
 
-A `switch` expression is similar to a `switch` statement, but you can use them
-anywhere you can use an expression. They produce a value based on the expression
-body of whichever case matches.
+A `switch` _expression_ produces a value based on the expression
+body of whichever case matches. 
+You can use a switch expression wherever Dart allows expressions,
+_except_ at the start of an expression statement. For example:
 
-The syntax of a `switch` expression differs from `switch` statement syntax:
+```dart
+var x = switch (y) { ... };
 
-- Cases _do not_ start with the `case` keyword.
-- A case body is a single expression instead of a series of statements.
-- Each case must have a body; there is no implicit fallthrough for empty cases.
-- Case patterns are separated from their bodies using `=>` instead of `:`.
-- Cases are separated by `,` (and an optional trailing `,` is allowed).
-- Default cases can _only_ use `_`, instead of allowing both `default` and `_`.
+print(switch (x) { ... });
+
+return switch (x) { ... };
+```
 
-Switch expressions allow you to rewrite a _statement_ like this:
+If you want to use a switch at the start of an expression statement,
+use a [switch statement](#switch-statements).
+
+Switch expressions allow you to rewrite a switch _statement_ like this:
 
 ```dart
 // Where slash, star, comma, semicolon, etc., are constant variables...
@@ -170,6 +173,15 @@ token = switch (charCode) {
 };
 ```
 
+The syntax of a `switch` expression differs from `switch` statement syntax:
+
+- Cases _do not_ start with the `case` keyword.
+- A case body is a single expression instead of a series of statements.
+- Each case must have a body; there is no implicit fallthrough for empty cases.
+- Case patterns are separated from their bodies using `=>` instead of `:`.
+- Cases are separated by `,` (and an optional trailing `,` is allowed).
+- Default cases can _only_ use `_`, instead of allowing both `default` and `_`.
+
 {{site.alert.version-note}}
     Switch expressions require a [language version][] of at least 3.0.
 {{site.alert.end}}