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.
Symbols highlighted with `highlight-symbol-mode' are added to
`highlight-symbol-list', which prevented them from being highlighted with
the command `highlight-symbol-at-point' because the function thought they
were already highlighted.
Fixed by adding another property, `:temp', to the pattern stored for
each symbol in `highlight-symbol-list' so that
`highlight-symbol-highlighted-p' can differentiate between the temporary
highlights created by `highlight-symbol-mode' and the more permanent
ones created by `highlight-symbol-at-point'.