Reorder and reword small portions of the keylength documentation and
make the ECC cert default explicitly stated in part 2 to avoid
confusion.
Fixes#6590
Useful where remote endpoints filter requests by IP address, but one's
Internet connection has a stable IP for only one address family,
e.g.: a dynamic IPv6 prefix and a static IPv4 address; or a static IPv6
prefix and CGNAT IPv4.