The docs were too specific. Make it vaguer. There are conditions for
which the Transport will try to reuse a connection anyway, even if the
Response Body isn't read to EOF or closed, but we don't need to get
into all the details in the docs.

Fixes #22954

Change-Id: I3b8ae32aeb1a61b396d0026e129552afbfecceec
Reviewed-on: https://go-review.googlesource.com/86276Reviewed-by: Ian Lance Taylor <iant@golang.org>