Warn on unescaped HTML-like markup

Comments like this:

```
// Blah blah blah
// projects//regions/
```

Can confuse the reference docs for C++. Doxygen considers `` to be an unknown HTML markup element. The fix is to use a code span:

```
// Blah blah blah
// `projects//regions/`
```

Which probably follows the style code.