Lifetime of CALLBACK undocumented

[till]

From looking at the (3.14.12) code it is apparent that the CALLBACK argument to 'callbackRequest(CALLBACK *pcbck)' is passed by reference. However, it is undocumented for how long the CALLBACK object is in 'possession' of the callback facility, i.e., when it is safe to re-use a static CALLBACK object or to free a dynamically created one. In particular, it should IMO be documented/defined if it is safe to reuse or free a CALLBACK from the callback function itself. It does seem so under the current (3.14.12) code which does not access CALLBACK after executing the associated function but it would IMO be nice to document the exact semantics.