Partial revert of rL260506.

After some experiments I discovered that clang doesn't support static
initialization of flexible array members in full, so restored this paragraph in
"GCC extensions not implemented yet" list.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260519 91177308-0d34-0410-b5e6-96231b3b80d8

docs/UsersManual.rst

@@ -1711,6 +1711,9 @@ extensions are not implemented yet:
      ...
      local_function(1);
 
+-  clang does not support static initialization of flexible array
+   members. This appears to be a rarely used extension, but could be
+   implemented pending user demand.
 -  clang does not support
    ``__builtin_va_arg_pack``/``__builtin_va_arg_pack_len``. This is
    used rarely, but in some potentially interesting places, like the