cppinternals.texi (Index): Fix html generation with makeinfo by renaming this node to Concept Index.

        * doc/cppinternals.texi (Index): Fix html generation with makeinfo by
        renaming this node to Concept Index.
        * doc/gccint.texi (Index): Likewise.  Also minor formatting tweaks
        to more closely match gcc.texi.

From-SVN: r90573