[thread.jthread.static] Repeat '[[nodiscard]]' from synopsis. (#3326)

Fixes NB JP 362 (C++20 CD)

Author: jensmaurer

source/threads.tex

@@ -1846,7 +1846,7 @@
 
 \indexlibrarymember{hardware_concurrency}{jthread}%
 \begin{itemdecl}
-unsigned hardware_concurrency() noexcept;
+[[nodiscard]] static unsigned int hardware_concurrency() noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}