CodingGuidelines: spell out post-C89 rules

Even though we have been sticking to C89, there are a few handy
features we borrow from more recent C language in our codebase after
trying them in weather balloons and saw that nobody screamed.

Spell them out.

While at it, extend the existing variable declaration rule a bit to
read better with the newly spelled out rule for the for loop.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -195,10 +195,30 @@ For C programs:
    by e.g. "echo DEVELOPER=1 >>config.mak".
 
  - We try to support a wide range of C compilers to compile Git with,
-   including old ones. That means that you should not use C99
-   initializers, even if a lot of compilers grok it.
+   including old ones.  You should not use features from newer C
+   standard, even if your compiler groks them.
 
- - Variables have to be declared at the beginning of the block.
+   There are a few exceptions to this guideline:
+
+   . since early 2012 with e1327023ea, we have been using an enum
+     definition whose last element is followed by a comma.  This, like
+     an array initializer that ends with a trailing comma, can be used
+     to reduce the patch noise when adding a new identifer at the end.
+
+   . since mid 2017 with cbc0f81d, we have been using designated
+     initializers for struct (e.g. "struct t v = { .val = 'a' };").
+
+   . since mid 2017 with 512f41cf, we have been using designated
+     initializers for array (e.g. "int array[10] = { [5] = 2 }").
+
+   These used to be forbidden, but we have not heard any breakage
+   report, and they are assumed to be safe.
+
+ - Variables have to be declared at the beginning of the block, before
+   the first statement (i.e. -Wdeclaration-after-statement).
+
+ - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
+   is still not allowed in this codebase.
 
  - NULL pointers shall be written as NULL, not as 0.