Use of the term "multipath"

[LPardue]

There's a few cases where we use the term "multipath" for either any transport or in relation to QUIC. We've just been through a bikeshed with this in https://github.com/quicwg/multipath/pull/613 (and the PR https://github.com/quicwg/multipath/pull/614). Once https://github.com/quicwg/qlog/pull/491/files lands, I think we should tweak the phrasing to something like "transports that use multiple paths" or equivalent.