changes: Document change in [nosplit reg]

Document that [nosplit reg] as opposed to [nosplit reg*1] will no
longer force an index register.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
doc: Document DEFAULT BND/NOBND in changes, add use case

Add DEFAULT BND/NOBND to the change history, and explain the use case.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
NASM 2.11