Improve comments / documentation about which section a Puppet environment should be set within the puppet.conf file

Puppet Version: 4.x and above
Puppet Server Version: NA
OS Name/Version: NA

There is some confusion around the proper place to set a Puppet environment in the puppet.conf file. An older (now fixed) bug (PUP-6739) highlighted problems with placing the environment in the [user] or [agent] section of the puppet.conf. Some documentation (e.g. https://puppet.com/docs/puppet/5.3/config_file_main.html#example-agent-config) shows an example of setting the environment in main, while the defaults.rb file under environment states to not set this globally.

Desired Behavior: Documentation about where to set the Puppet environment in the puppet.conf should be consistent with the code base and what is recommended as well as other existing documentation.  **

Actual Behavior:

Defaults.rb has this in the comments under environment:

"Given that the context and effects vary depending on the [config section](https://puppet.com/docs/puppet/latest/config_file_main.html#config-sections) in which the `environment` setting is defined, do not set it globally."