doc: tee -p: clarify operation

doc: tee -p: clarify operation

* src/tee.c (usage): Change from describing one (non pipe) aspect
to the more general point of being the option to use if working with
pipes, and referencing the more detailed info below.
* doc/coreutils.texi (tee invocation): s/standard/appropriate/ since
the standard operation with pipes is to exit immediately upon write
error.  s/early/immediately/ as it's ambiguous as to what "early"
is in relation to.