Bring back note about not supporting global register variables

[mkuper]

Summary

This partially reverts r260506 which removed this note.

Renato, the part about "not being implemented soon because it requires backend support" was taken verbatim from the original text, pre-r260506, I only added the fact non-allocatable registers are now supported.
If you prefer to have strong wording, feel free to make suggestions.

[rengolin]

LGTM. Thanks!

docs/UsersManual.rst

-  clang does not support nested functions; this is a complex feature
    .. code-block:: cpp
 
      auto const local_function = [&](int parameter) {
        // Do something
      };
      ...
      local_function(1);
 
+-  clang only supports global register variables when the register specified
+   is non-allocatable (e.g. the stack pointer). Support for general global
+   register variables is unlikely to be implemented soon because it requires
+   additional LLVM backend support.
 -  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
    glibc headers, so it may be implemented pending user demand. Note
    that because clang pretends to be like GCC 4.2, and this extension