CodingGuidelines: describe "export VAR=VAL" rule

https://lore.kernel.org/git/201307081121.22769.tboegi@web.de/
resulted in 9968ffff (test-lint: detect 'export FOO=bar',
2013-07-08) to add a rule to t/check-non-portable-shell.pl script to
reject

	export VAR=VAL

and suggest us to instead write it as two statements, i.e.,

	VAR=VAL
	export VAR

This however was not spelled out in the CodingGuidelines document.

We may want to re-evaluate the rule since it is from ages ago, but
for now, let's make the written rule and what the automation
enforces consistent.

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

Documentation/CodingGuidelines

@@ -188,6 +188,10 @@ For shell scripts specifically (not exhaustive):
    hopefully nobody starts using "local" before they are reimplemented
    in C ;-)
 
+ - Some versions of shell do not understand "export variable=value",
+   so we write "variable=value" and then "export variable" on two
+   separate lines.
+
  - Use octal escape sequences (e.g. "\302\242"), not hexadecimal (e.g.
    "\xc2\xa2") in printf format strings, since hexadecimal escape
    sequences are not portable.