Patch to merge objc manual from mainline

[Nicola Pero]

>     Joseph> It was previously discussed that @compatibility_alias had
>     Joseph> existed in the objc compiler for a long time and so the
>     Joseph> objc manual could be the same on mainline and branch.  The
>     Joseph> following patch effects that change.  It passes "make
>     Joseph> info" and "make dvi"; OK to commit to 3.0 branch?
> 
> Stan needs to comment on the overall correctness.
> 
> +This is a feature of the Objective-C compiler rather than of the
> +runtime, anyway since it is documented nowhere and its existence was
> +forgotten, we are documenting it here.
> 
> This sounds like it should maybe go in extend.texi?  We don't need
> this kind of paragraph; we've explored our cutely quirky sides enough
> in our documentation already. :-)

Well - yes - extend.texi sounds good for that ...

If I get some more time, I wouldn't mind documenting a bit more of our
Objective-C compiler and language - but I'm not sure where that would go -
not sure it should go into extend.texi, because it's difficult to decide
what is an `extension' made by the GCC Objective-C compiler and what is
just `standard' Objective-C: basically as far as I understand and feel,
the GCC Objective-C compiler is currently sort-of the mainstream
Objective-C compiler, it has probably more of 90% of Objective-C
programmers `marketshare', and it is - de facto - the reference
implementation of the Objective-C language.  I'm quite proud it's GNU :-)

But yes, @compatibility_alias is definitely an extension.