doc: formatting fixes to the warnings documentation
doc: formatting fixes to the warnings documentation

Formatting and language consistency cleanups to the sections about
disabling and enabling warning classes.

Signed-off-by: H. Peter Anvin <8a453bad9912ffe59bc0f0b8abe03df9be19379e@zytor.com>

techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm