[doc PATCH] document variable attribute alias

[Martin Sebor]

It was pointed out recently in another forum that GCC doesn't
document attribute alias for variables.  It was also noted in
the same discussion that the semantics of accessing aliases
and their targets can have "surprising" effects because GCC
(as do other compilers) assumes that distinct declarations
with external linkage denote distinct entities.

The attached patch adds text to the manual describing attribute
alias for variables and pointing out the caveat of accessing
the same object using both the alias and the target.

Martin

PS Unlike for function aliases where type mismatches have been
diagnosed by -Wattribute-alias since 8.1, GCC doesn't complain
when the the type of an alias target variable doesn't match that
of its aliases.  Except for top-level qualifiers where it might
make sense to allow the target to be less qualified than
the alias (similarly as with const pointers to non-const objects
or with C++ const references), I assume it's just an oversight.
I will try to look into diagnosing these mismatches as well.
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