docs: remove mention of ENABLE_MUST_CHECK

docs: remove mention of ENABLE_MUST_CHECK

We removed ENABLE_MUST_CHECK in 196793946264 ("Compiler Attributes:
remove CONFIG_ENABLE_MUST_CHECK"), so let's remove docs' mentions.

At the same time, fix the outdated text related to
ENABLE_WARN_DEPRECATED that wasn't removed in 3337d5cfe5e08
("configs: get rid of obsolete CONFIG_ENABLE_WARN_DEPRECATED").

Finally, reflow the paragraph.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20210105055815.GA5173@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>