Don't require comments to start with double asterisks to be docs

Currently this is picked up as documentation (`haxe -xml doc.xml -D doc-gen`, for use with dox)

```
/**
Foo
**/
```

but this isn't

```
/*
Foo
*/
```

I remember this to be somewhat intentional in the old days (although I couldn't find any reference to it), but it still seems [somewhat arbitrary](https://github.com/HaxeFoundation/haxe/blob/d5daf4f/src/syntax/parserEntry.ml#L91) and, also, not documented anywhere.

Wouldn't treating any `Comment` (!= `CommentLine`) as valid documentation be more uniform and straightforward?