bug#46702: Link symbol name in Help

[Peter Dean]

When I wanted to a check a buffer's mode then I did a C-h v
major-mode to see which mode it is and wanted to jump to its code
to check something. The symbol value was not linked, so I had to
type it to look it up.

Demonstrated here with the scratch buffer, showing that
lisp-interaction-mode is not linked in the second line:

   https://i.imgur.com/KIWQE60.png

It could link to the symbol's help if the value is a symbol to
make looking it up more convenient.

If it's both a function and a variable then it can ask which one
I want to jump to when clicked.