[libcxx][docs] Mark LWG3356 as complete

Feature test macro for `__cpp_lib_is_nothrow_convertible` was introduced in
466df17 but the LWG issue was not marked as
`Complete` in the docs. Also, fix the formatting of `Complete` for
LWG 3348.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D108964

Author: JoeLoser

libcxx/docs/Status/Cxx20Issues.csv

@@ -259,14 +259,14 @@
 "`3340 <https://wg21.link/LWG3340>`__","Formatting functions should throw on argument/format string mismatch in |sect|\ [format.functions]","Prague","",""
 "`3346 <https://wg21.link/LWG3346>`__","``pair``\  and ``tuple``\  copy and move constructor have backwards specification","Prague","",""
 "`3347 <https://wg21.link/LWG3347>`__","``std::pair<T, U>``\  now requires ``T``\  and ``U``\  to be less-than-comparable","Prague","",""
-"`3348 <https://wg21.link/LWG3348>`__","``__cpp_lib_unwrap_ref``\  in wrong header","Prague","Complete","12.0"
+"`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","",""
 "`3350 <https://wg21.link/LWG3350>`__","Simplify return type of ``lexicographical_compare_three_way``\ ","Prague","",""
 "`3351 <https://wg21.link/LWG3351>`__","``ranges::enable_safe_range``\  should not be constrained","Prague","",""
 "`3352 <https://wg21.link/LWG3352>`__","``strong_equality``\  isn't a thing","Prague","",""
 "`3354 <https://wg21.link/LWG3354>`__","``has_strong_structural_equality``\  has a meaningless definition","Prague","",""
 "`3355 <https://wg21.link/LWG3355>`__","The memory algorithms should support move-only input iterators introduced by P1207","Prague","",""
-"`3356 <https://wg21.link/LWG3356>`__","``__cpp_lib_nothrow_convertible``\  should be ``__cpp_lib_is_nothrow_convertible``\ ","Prague","",""
+"`3356 <https://wg21.link/LWG3356>`__","``__cpp_lib_nothrow_convertible``\  should be ``__cpp_lib_is_nothrow_convertible``\ ","Prague","|Complete|","12.0"
 "`3358 <https://wg21.link/LWG3358>`__","|sect|\ [span.cons] is mistaken that ``to_address``\  can throw","Prague","",""
 "`3359 <https://wg21.link/LWG3359>`__","``<chrono>``\  leap second support should allow for negative leap seconds","Prague","",""
 "`3360 <https://wg21.link/LWG3360>`__","``three_way_comparable_with``\  is inconsistent with similar concepts","Prague","",""