compiler: handle switch rewrite review feedback

doc/langref.html.in

@@ -2595,7 +2595,7 @@ or
 
       {#header_close#}
 
-      {#header_open|Switching on errors#}
+      {#header_open|Switching on Errors#}
       <p>
       When switching on errors, some special cases are allowed to simplify generic programming patterns:
       </p>

doc/langref/test_switch_on_errors.zig

@@ -12,7 +12,10 @@ test "unreachable else prong" {
     switch (openFile0()) {
         error.AccessDenied, error.FileNotFound => |e| return e,
         error.OutOfMemory => {},
-        else => unreachable, // technically unreachable, but will still compile!
+        // 'openFile0' cannot return any more errors, so an 'else' prong would be
+        // statically known to be unreachable. Nonetheless, in this case, adding
+        // one does not raise an "unreachable else prong" compile error:
+        else => unreachable,
     }
 
     // Allowed unreachable else prongs are:

doc/langref/test_tagged_union.zig

@@ -20,7 +20,10 @@ test "switch on tagged union" {
     }
 
     switch (c) {
-        .ok => |_, tag| try expect(tag == .ok),
+        .ok => |_, tag| {
+            // Because we're in the '.ok' prong, 'tag' is compile-time known to be '.ok':
+            comptime std.debug.assert(tag == .ok);
+        },
         .not_ok => unreachable,
     }
 }