Merge branch 'jc/document-use-of-local'

Doc update.

* jc/document-use-of-local:
  doc: note that AT&T ksh does not work with our test suite

Documentation/CodingGuidelines

@@ -185,8 +185,8 @@ For shell scripts specifically (not exhaustive):
 
  - Even though "local" is not part of POSIX, we make heavy use of it
    in our test suite.  We do not use it in scripted Porcelains, and
-   hopefully nobody starts using "local" before they are reimplemented
-   in C ;-)
+   hopefully nobody starts using "local" before all shells that matter
+   support it (notably, ksh from AT&T Research does not support it yet).
 
  - Some versions of shell do not understand "export variable=value",
    so we write "variable=value" and then "export variable" on two