docs: remove mention of ENABLE_MUST_CHECK

We removed ENABLE_MUST_CHECK in 19679394 ("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At the same time, fix the outdated text related to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 3337d5cf
("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.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/4.Coding.rst

@@ -249,10 +249,8 @@ features; most of these are found in the "kernel hacking" submenu.  Several
 of these options should be turned on for any kernel used for development or
 testing purposes.  In particular, you should turn on:
 
- - ENABLE_MUST_CHECK and FRAME_WARN to get an
-   extra set of warnings for problems like the use of deprecated interfaces
-   or ignoring an important return value from a function.  The output
-   generated by these warnings can be verbose, but one need not worry about
+ - FRAME_WARN to get warnings for stack frames larger than a given amount.
+   The output generated can be verbose, but one need not worry about
    warnings from other parts of the kernel.
 
  - DEBUG_OBJECTS will add code to track the lifetime of various objects