(MAINT) Update tuning guide to be explicit about 2.0 defaults
(MAINT) Update tuning guide to be explicit about 2.0 defaults

This commit updates the tuning guide doc to be more explicit
about the default values for `max-active-instances`, including
an explicit description of the behavior in 2.0.0.

cgvarela/puppet-server,shrug/puppet-server,er0ck/puppetserver,camlow325/puppet-server,jeffmccune/puppet-server,puppetlabs/puppetserver,kylog/puppet-server,stahnma/puppet-server,er0ck/puppetserver,er0ck/puppetserver,waynr/puppet-server,waynr/puppetserver,dankreek/puppet-server,waynr/puppet-server,ahpook/puppet-server,camlow325/puppetserver,cprice404/puppet-server,puppetlabs/puppetserver,dankreek/puppet-server,dankreek/puppet-server,rlinehan/puppetserver,cprice404/puppet-server,camlow325/puppet-server,erikPrime/puppetserver,shrug/puppet-server,puppetlabs/puppet-server,puppetlabs/puppetserver,erikPrime/puppetserver,erikPrime/puppet-server,cgvarela/puppet-server,camlow325/puppetserver,rlinehan/puppetserver,puppetlabs/puppet-server,waynr/puppetserver,waynr/puppetserver,fpringvaldsen/puppet-server,cprice404/puppetserver,puppetlabs/puppetserver,rlinehan/puppetserver,rlinehan/puppet-server,cprice404/puppet-server,fpringvaldsen/puppet-server,ahpook/puppet-server,cprice404/puppetserver,waynr/puppet-server,stahnma/puppet-server,cgvarela/puppet-server,kylog/puppet-server,rlinehan/puppetserver,jeffmccune/puppet-server,cprice404/puppetserver,briancain/puppet-server,camlow325/puppetserver,rlinehan/puppet-server,shrug/puppet-server,ahpook/puppet-server,rlinehan/puppet-server,stahnma/puppet-server,erikPrime/puppet-server,erikPrime/puppet-server,jeffmccune/puppet-server,er0ck/puppetserver,camlow325/puppet-server,fpringvaldsen/puppet-server,briancain/puppet-server,kylog/puppet-server,briancain/puppet-server,shrug/puppet-server,puppetlabs/puppet-server,camlow325/puppetserver,erikPrime/puppetserver