fix: differentiate between regular and top-level document comments in error messages

doc/langref/invalid_doc-comment.zig

@@ -2,4 +2,4 @@
 //! top-level doc-comment
 const std = @import("std");
 
-// obj=expected type expression, found 'a document comment'
+// obj=expected type expression, found 'a top-level document comment'

lib/std/zig/tokenizer.zig

@@ -318,7 +318,8 @@ pub const Token = struct {
                 .eof => "EOF",
                 .builtin => "a builtin function",
                 .number_literal => "a number literal",
-                .doc_comment, .container_doc_comment => "a document comment",
+                .doc_comment => "a document comment",
+                .container_doc_comment => "a top-level document comment",
                 else => unreachable,
             };
         }

lib/std/zon/parse.zig

@@ -1306,7 +1306,7 @@ test "std.zon comments" {
             \\// comment
         , &diag, .{}));
         try std.testing.expectFmt(
-            "1:1: error: expected expression, found 'a document comment'\n",
+            "1:1: error: expected expression, found 'a top-level document comment'\n",
             "{f}",
             .{diag},
         );

test/cases/compile_errors/@import_zon_doc_comment.zig

@@ -6,4 +6,4 @@ export fn entry() void {
 // error
 // imports=zon/doc_comment.zon
 //
-// doc_comment.zon:1:1: error: expected expression, found 'a document comment'
+// doc_comment.zon:1:1: error: expected expression, found 'a top-level document comment'

test/cases/compile_errors/top_level_doc_comment.zig

@@ -0,0 +1,8 @@
+pub const foo = 5;
+
+//! This is main.
+pub fn main() void {}
+
+// error
+//
+// 1:19: error: expected type expression, found 'a top-level document comment'