doc: specify C99 coding standards

Make clear that Tor's C code targets C99 standards. This makes it more
explicit what to expect for new code, because guessing from existing
code is not always reliable, especially for code that predates the
change in standard.

doc/HACKING/CodingStandards.md

@@ -193,8 +193,9 @@ Why use changes files instead of entries in the ChangeLog?
 
 ## Whitespace and C conformance
 
-Invoke `make check-spaces` from time to time, so it can tell you about
-deviations from our C whitespace style.  Generally, we use:
+Tor's C code is written in accordance with the C99 standard. Invoke `make
+check-spaces` from time to time, so it can tell you about deviations from our C
+whitespace style.  Generally, we use:
 
    - Unix-style line endings
    - K&R-style indentation
@@ -213,6 +214,8 @@ deviations from our C whitespace style.  Generally, we use:
    - Use `void foo(void)` to declare a function with no arguments.  Saying
      `void foo()` is C++ syntax.
    - Use `const` for new APIs.
+   - Variables should be initialized when declared, rather than declared at the
+     top of a scope.
 
 If you use an editor that has plugins for editorconfig.org, the file
 `.editorconfig` will help you to conform this coding style.