Documentation/process: Add fallthrough pseudo-keyword

Describe the fallthrough pseudo-keyword.

Convert the coding-style.rst example to the keyword style.
Add description and links to deprecated.rst.

Miguel Ojeda comments on the eventual [[fallthrough]] syntax:
 "Note that C17/C18 does not have [[fallthrough]].

  C++17 introduced it, as it is mentioned above. I would keep the
  __attribute__((fallthrough)) -> [[fallthrough]] change you did,
  though, since that is indeed the standard syntax (given the paragraph
  references C++17).

  I was told by Aaron Ballman (who is proposing them for C) that it is
  more or less likely that it becomes standardized in C2x. However, it
  is still not added to the draft (other attributes are already,
  though). See N2268 and N2269:

     http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2268.pdf (fallthrough)
     http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2269.pdf (attributes in general)"
Signed-off-by: NJoe Perches <joe@perches.com>
Acked-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/process/coding-style.rst

@@ -56,7 +56,7 @@ instead of ``double-indenting`` the ``case`` labels.  E.g.:
 	case 'K':
 	case 'k':
 		mem <<= 10;
-		/* fall through */
+		fallthrough;
 	default:
 		break;
 	}

Documentation/process/deprecated.rst

@@ -122,14 +122,27 @@ memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
 
 Implicit switch case fall-through
 ---------------------------------
-The C language allows switch cases to "fall through" when
-a "break" statement is missing at the end of a case. This,
-however, introduces ambiguity in the code, as it's not always
-clear if the missing break is intentional or a bug. As there
-have been a long list of flaws `due to missing "break" statements
+The C language allows switch cases to "fall-through" when a "break" statement
+is missing at the end of a case. This, however, introduces ambiguity in the
+code, as it's not always clear if the missing break is intentional or a bug.
+
+As there have been a long list of flaws `due to missing "break" statements
 <https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
-"implicit fall-through". In order to identify an intentional fall-through
-case, we have adopted the marking used by static analyzers: a comment
-saying `/* Fall through */`. Once the C++17 `__attribute__((fallthrough))`
-is more widely handled by C compilers, static analyzers, and IDEs, we can
-switch to using that instead.
+"implicit fall-through".
+
+In order to identify intentional fall-through cases, we have adopted a
+pseudo-keyword macro 'fallthrough' which expands to gcc's extension
+__attribute__((__fallthrough__)).  `Statement Attributes
+<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_
+
+When the C17/C18  [[fallthrough]] syntax is more commonly supported by
+C compilers, static analyzers, and IDEs, we can switch to using that syntax
+for the macro pseudo-keyword.
+
+All switch/case blocks must end in one of:
+
+	break;
+	fallthrough;
+	continue;
+	goto <label>;
+	return [expression];