[doc] Document that __fp16 will apply default argument promotion rule.

[kito-cheng]

Summary

According Procedure Call Standard for the ARM 64-bit Architecture, __fp16 has specified it will promote to double when passed to unprototyped and variadic.

[kito-cheng]

Update SUMMARY, first version I just incorrectly said ACLE didn't clarify the default argument promotion rule, but I found that in ARM ABI spec later.

clang/docs/LanguageExtensions.rst

 Because ``__fp16`` operands are always immediately promoted to ``float``, the
 common real type of ``__fp16`` and ``_Float16`` for the purposes of the usual
 arithmetic conversions is ``float``.
 
 A literal can be given ``_Float16`` type using the suffix ``f16``. For example,
 ``3.14f16``.
 
 Because default argument promotion only applies to the standard floating-point
-types, ``_Float16`` values are not promoted to ``double`` when passed as variadic
-or untyped arguments.  As a consequence, some caution must be taken when using
-certain library facilities with ``_Float16``; for example, there is no ``printf`` format
-specifier for ``_Float16``, and (unlike ``float``) it will not be implicitly promoted to
-``double`` when passed to ``printf``, so the programmer must explicitly cast it to
-``double`` before using it with an ``%f`` or similar specifier.
+types and ``__fp16``, ``_Float16`` values are not promoted to ``double`` when
+passed as variadic or untyped arguments.  As a consequence, some caution must
+be taken when using certain library facilities with ``_Float16``; for example,
+there is no ``printf`` format specifier for ``_Float16``, and (unlike ``float``)
+it will not be implicitly promoted to ``double`` when passed to ``printf``, so
+the programmer must explicitly cast it to ``double`` before using it with an
+``%f`` or similar specifier.
 
 Messages on ``deprecated`` and ``unavailable`` Attributes
 =========================================================
 
 An optional string message can be added to the ``deprecated`` and
 ``unavailable`` attributes.  For example:
 
 .. code-block:: c++