The check for sslsni only checked for existence of the parameter
but not for the actual value of the param.  This meant that the
SNI extension was always turned on.  Fix by inspecting the value
of sslsni and only activate the SNI extension iff sslsni has been
enabled.  Also update the docs to be more in line with how other
boolean params are documented.

Backpatch to 14 where sslsni was first implemented.

Reviewed-by: Tom Lane
Backpatch-through: 14, where sslni was added