bug#4257: Documentation for where-is-preferred-modifier is obscure

[Alan Mackenzie]

The doc string for `where-is-preferred-modifier' is

   where-is-preferred-modifier is a variable defined in `C source code'.
   Its value is nil

   Documentation:
   Preferred modifier to use for `where-is'.
   When a single binding is requested, `where-is' will return one that
   uses this modifier if possible.  If nil, or if no such binding exists,
   bindings using keys without modifiers (or only with meta) will be
   preferred.

.  This leaves the user having to guess what a "modifier for `where-is'"
is.  This is bad.

The same obscurity exists in NEWS.  This is somewhat bad.

The variable is not documented in the Emacs manual.  This might be bad.

-- 
Alan Mackenzie (Nuremberg, Germany).