Outdated comment in lisp.h

[Eli Zaretskii]

This comment:

  /* Value of the symbol or Qunbound if unbound.  If this symbol is a
     defvaralias, `alias' contains the symbol for which it is an
     alias.  Use the SYMBOL_VALUE and SET_SYMBOL_VALUE macros to get
     and set a symbol's value, to take defvaralias into account.  */

is false in its last sentence, since the macros it refers to no longer
exist.  It's not clear to me whether replacements exist; someone who
is "in the know" please fix the comment as appropriate.