[libc++][doc] Updates LWG3346 status. (#80536)

The issue addresses an obvious wording issue. Implementing the
constructors as specified in the synposis, as libc++ did, already
implements the fixed behaviour.

Updates:
- LWG3346 pair and tuple copy and move constructor have backwards
specification

Author: mordante

libcxx/docs/Status/Cxx20Issues.csv

@@ -258,7 +258,7 @@
 "`3335 <https://wg21.link/LWG3335>`__","Resolve C++20 NB comments US 273 and GB 274","Prague","|Complete|","15.0","|ranges|"
 "`3338 <https://wg21.link/LWG3338>`__","Rename ``default_constructible``\  to ``default_initializable``\ ","Prague","|Complete|","13.0"
 "`3340 <https://wg21.link/LWG3340>`__","Formatting functions should throw on argument/format string mismatch in |sect|\ [format.functions]","Prague","|Complete|","14.0","|format|"
-"`3346 <https://wg21.link/LWG3346>`__","``pair``\  and ``tuple``\  copy and move constructor have backwards specification","Prague","",""
+"`3346 <https://wg21.link/LWG3346>`__","``pair``\  and ``tuple``\  copy and move constructor have backwards specification","Prague","|Nothing To Do|",""
 "`3347 <https://wg21.link/LWG3347>`__","``std::pair<T, U>``\  now requires ``T``\  and ``U``\  to be less-than-comparable","Prague","|Complete|","17.0"
 "`3348 <https://wg21.link/LWG3348>`__","``__cpp_lib_unwrap_ref``\  in wrong header","Prague","|Complete|","12.0"
 "`3349 <https://wg21.link/LWG3349>`__","Missing ``__cpp_lib_constexpr_complex``\  for P0415R1","Prague","|Complete|","16.0"