[func.wrap.move.ctor] Move private expos member to general index

The private, exposition-only data member is not useful in the library
index. However, there seems no reason to not show it in the general
index, so we move the index entry from the library index to the
general index.

Partially fixes NB GB 137 (C++23 CD).

Author: tkoeppe

source/utilities.tex

@@ -12424,7 +12424,7 @@
 
 \rSec4[func.wrap.move.ctor]{Constructors, assignment, and destructor}
 
-\indexlibrarymember{\exposid{is-callable-from}}{move_only_function}%
+\indextext{move_only_function::is-callable-from@\tcode{move_only_function::\exposid{is-callable-from}}}%
 \begin{itemdecl}
 template<class VT>
   static constexpr bool @\exposid{is-callable-from}@ = @\seebelow@;