[macros] add \libspec macro to index specializations

`\libspec{disable_sized_sentinel_for}{move_iterator}`
will index `move_iterator` under `disable_sized_sentinel_for`,
which is enough to uniquely identify
```C++
  template<class Iterator1, class Iterator2>
      requires (!sized_sentinel_for<Iterator1, Iterator2>)
    constexpr bool disable_sized_sentinel_for<move_iterator<Iterator1>,             // freestanding
                                              move_iterator<Iterator2>> = true;
```

Author: JohelEGP

source/macros.tex

@@ -205,6 +205,7 @@
 
 \newcommand{\libglobal}[1]{\indexlibraryglobal{#1}#1}
 \newcommand{\libmember}[2]{\indexlibrarymember{#1}{#2}#1}
+\newcommand{\libspec}[2]{\indexlibrarymemberx{#1}{#2}#1}
 
 % index for library headers
 \newcommand{\libheader}[1]{\indexhdr{#1}\tcode{<#1>}}