Clarify claim about non-identifying data in --phone_home docs

The `--phone_home` option documentation implied that the report only
contains non-identifying information.  However, the official forum page
for the Phone Home feature explaining what data is sent includes an
example XML payload with user and host names (and indeed the
GATKRunReport code supports this).  Clarify for users that some
potentially (likely, even) identifying information is sent.