Merge branch 'jk/doc-do-not-write-extern'

Devdoc update.

* jk/doc-do-not-write-extern:
  CodingGuidelines: mention "static" and "extern"

Documentation/CodingGuidelines

@@ -386,6 +386,11 @@ For C programs:
  - Use Git's gettext wrappers to make the user interface
    translatable. See "Marking strings for translation" in po/README.
 
+ - Variables and functions local to a given source file should be marked
+   with "static". Variables that are visible to other source files
+   must be declared with "extern" in header files. However, function
+   declarations should not use "extern", as that is already the default.
+
 For Perl programs:
 
  - Most of the C guidelines above apply.