It is common for users to wish to adjust the verification_mode in SSL
settings, usually with the intention of skipping hostname
verification. This has been supported for a long time, but the
relevant configuration setting was not clearly documented, which would
sometimes lead users to set `verification_mode` to `none`, and disable
more checks than they intended.
This commit adds clearer documentation regarding the options available
for `verification_mode` and actively discourages the use of `none`.
Original commit: elastic/x-pack-elasticsearch@2fdf53b42f
411f683521