doc: give a tee example for combining process substitution outputs

doc: give a tee example for combining process substitution outputs

This can be useful if you want to further process data
from process substitutions. For example:
  datagen | tee >(md5sum --tag) > >(sha256sum --tag) | sort

* doc/coreutils.texi (tee invocation): Mention that -p is
useful with pipes that may not consume all data.
Add an example, similar to the one above.
* THANKS.in: Add Jirka Hladky.