[PATCH, Fortran] Internal documentation for gfc_expr

[Daniel Kraft]

Hi,

attached is a patch and changelog that creates a new section "gfc_expr" 
in the "Front-end data structures" chapter of gfc-internals.texi 
describing how expressions are structured.

As usual there are some XXX comments in that I'll remove in any case 
before check-in and where I'd like to get special comments / special 
care in review.

During my work on the finalization-patch the two main points where I 
missed documentation about gfc_expr were construction of intrinsic-calls 
and how the ref-list of EXPR_VARIABLE expressions works exactly; while 
I've not yet found out something really useful about the first one, I 
tried to document the latter from what I found out detailed.  Most other 
parts of the documentation are somewhat trivial but I included them for 
completeness' sake (and maybe they are not that clear to every reader).

Once again, I'm not sure about some things myself, so I'd love to get 
critical review from someone who knows more than I do.  I'll check the 
(corrected) patch in when I get ok.

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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