[PATCH, Fortran] Extend gfc_code internal documentation

[Daniel Kraft]

Hi,

this patch extends the documentation of the gfc_code structure in the 
gfc-internals.texi documentation.  It adds an example about how the 
block-chaining for IF/SELECT works and has three detail subsections 
about DO, IF and SELECT blocks.

Please give it a close look as I'm myself rather unsure about the things 
I wrote about...  And there are two XXX marks in, please comment on 
those, too.  Otherwise I think it should be faily simple and if it's ok 
I'll check it in.  Then I can start writing a bit about gfc_expr in a 
new following section.

Daniel

-- 
Done:     Bar-Sam-Val-Wiz, Dwa-Elf-Hum-Orc, Cha-Law, Fem-Mal
Underway: Ran-Gno-Neu-Fem
To go:    Arc-Cav-Hea-Kni-Mon-Pri-Rog-Tou
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