Patch to document ISALPHA etc. convention

[Mark Mitchell]

Joseph S. Myers wrote:
> Recent discussion e.g. 
> <http://gcc.gnu.org/ml/fortran/2005-01/msg00084.html> illustrated that the 
> use of ISALPHA etc. in place of isalpha etc. isn't documented.  This patch 
> adds an explanation in codingconventions.html.  OK to commit?

Yes.

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Mark Mitchell
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