Use `font-lock' functions instead of `hi-lock-mode'

Using 'hi-lock-mode' made the foreground color of all highlighted symbols black, ignoring the underlying setting.  Directly using the 'font-lock' functions avoids that issue.

Also clarified various docstrings and added the functions 'highlight-symbol-add', 'highlight-symbol-remove', and 'highlight-symbol-highlighted-p' to highlight/unhighlight individual symbols and check if symbols are highlighted.