coreutils.texi node comm/ptx/tsort node ordering

[Karl Berry]

Either the menu or the node ordering should be changed.
And the tsort background node should be mentioned, or perhaps made a
subsection of tsort invocation.

* comm invocation::             Compare two sorted files line by line.
* ptx invocation::              Produce a permuted index of file contents.
* tsort invocation::            Topological sort.
...
@node comm invocation
...
@node tsort invocation
...
@node tsort background
...
@node ptx invocation

Perhaps makeinfo should warn about such mismatches, but meanwhile ...

karl