Doc: get rid of <foreignphrase> tags.

Doc: get rid of <foreignphrase> tags.

We italicized some, but not all, instances of "per se", "pro forma", and
"ad hoc". These phrases are widespread in formal registers of English,
so it"s debatable whether they even qualify as foreign. We could instead
try to be more consistent in the use of <foreignphrase>, but that"s
difficult to enforce, so let"s just remove the tags for those words.

The one case that seems to deserve the tag is "voilà". Instead of keeping
just one instance of the tag, change that to a more standard phrase.

John Naylor

Discussion: https://postgr.es/m/CAFBsxsHtWs_NsccAVgQ=tTUKkXHpHdkjZXtp_Cd9dGWyBDxfbQ@mail.gmail.com