install documentation updates

[James E Wilson <wilson at specifixinc dot com>]

This is an attempt to fix the install doc bugs reported at the end of
    http://gcc.gnu.org/ml/gcc/2005-04/msg01258.html

This patch drops f77 from the list of supported languages, adds all and
treelang, and documents the default languages.  There was a @* line
break that didn't seem to serve any purpose, so I dropped it.

In the gcc-4.0 patch, I also drop obj-c++ as a language.  I left it in
mainline, as Mike Stump is actually trying to add the language now. 
This is the only difference between the two patches.

This also fixes the path to the gcc html docs.

I think this is all pretty obvious except perhaps adding treelang to the
docs, which might prompt questions about what it is, but still, it is
valid, and so it should be listed.

This was tested with make info and make dvi on mainline and the gcc-4.0
branch.

make dvi failed on mainline because of a problem with gcc.texi that I
didn't touch.  It complained about a spurious group-closing symbol "}",
but gave no helpful info like a line number, so I don't know offhand
what to do about that.

Comments anyone?  If I don't get any comments in a few days, I will go
ahead and check this in.
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com

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