vhdl-symbol-key is a variable defined in `vhdl-mode.el'.
Its value is
"\\(\\w\\|\\s_\\)+"
This variable may be risky if used as a file-local variable.
Documentation:
Regexp describing a VHDL symbol.
We cannot use just `word' syntax class since `_' cannot be in word
class. Putting underscore in word class breaks forward word movement
behavior that users are familiar with.