CPP13 — ValueBase::_copy_value clarification

Summary: The ValueBase::_copy_value() function is included in the discussion
of the "reference counting interface" in section 1.17.5 of 99-07-41.
Later, the description of the reference counting mix-in classes
says:

"Each of these classes shall be fully concrete and shall completely
fulfill the ValueBase reference counting interface..."

However, I do not believe that the intent was to require the mix-in
classes to implement _copy_value().

Therefore I suggest one of two clarifications:

1) Move the discussion of _copy_value() out of the reference-counting
section, or

2) Specify which functions the mix-in classes are actually expected to
implement, e.g.,

"Each of these classes shall be fully concrete and shall completely
fulfill the ValueBase reference counting interface (_add_ref,
_remove_ref, and _refcount_value), ..."