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

[WillLillis]

Given:

pub const foo = 5;

//! This is main.
pub fn main() void {}

Before:

1:19: error: expected type expression, found 'a document comment'

After:

1:19: error: expected type expression, found 'a top-level document comment'

Tested locally with stage4/bin/zig build test -Dskip-non-native

• Closes Compile error for misplaced top-level documentation comment is misleading #21793