Also introduce the descriptive mechanism of an
exposition-only function.
Remove [thread.decaycopy] by moving its contents here.
Use the spelling 'decay-copy' (italics code font) instead of
introducing an intermediate meta-macro DECAY_COPY.

not just to capture language linkage.  For example,
'node-handle' is an entire exposition-only class.

This subclause contains many normative definitions
(e.g. [operators]); marking it as 'informative' is
misleading.