Fix documentation of completion functions

So that the described behavior matches the code (and docstrings).

* doc/lispref/minibuf.texi (Basic Completion): Symbol keys of hash table
collections are used like string keys, not ignored (Bug #10416).

doc/lispref/minibuf.texi

@@ -733,8 +733,8 @@ list contains elements of any other type, those are ignored.
 If @var{collection} is an obarray (@pxref{Creating Symbols}), the names
 of all symbols in the obarray form the set of permissible completions.
 
-If @var{collection} is a hash table, then the keys that are strings
-are the possible completions.  Other keys are ignored.
+If @var{collection} is a hash table, then the keys that are strings or
+symbols are the possible completions.  Other keys are ignored.
 
 You can also use a function as @var{collection}.  Then the function is
 solely responsible for performing completion; @code{try-completion}