In the current documentation is the C API function TSIOBufferReaderRead which
reads from an IOBufferReader in to a fixed buffer, the dual of
TSIOBufferWriter. This function is not actually implemented which has caused
some consternation. In working on providing an implementation, I came across an
implementation for TSIOBufferReaderCopy which does (almost) the same thing but
for unknown reasons was hidden from the C API. I have PR #5668 which unifies
these, keeping the name TSIOBufferReaderCopy, cleaning up the implementation,
and updating the documentation. It's somewhat unclear if this is really "new"
API since a variant already existed and the non-existent variant was already
documented, but might as well be clear and proper about it.
