The section on buffer synchronization rules define the scenarios when
the buffer destructor must block vs. the scenarios where it does not
block.  The old wording made it seem like a buffer created from no host
pointer is only non-blocking if it uses the default memory allocator.
I assume this was not our intent, and such a buffer should have a
non-blocking destructor regardless of the memory allocator.

The main change in this PR is to remove the phrase "and using the
default buffer allocator".  However, I also reworded a bit to clarify
which type of buffers we are talking about.

Avoid the word "mapped" and say "used to construct" instead.

Clarify that the implementation may keep a copy of a buffer allocator
even after the buffer is destroyed.

Co-authored-by: Ronan Keryell <[email protected]>