update documentation for generate function to clarify possible return values

the generate function returns the total command output of a command including any meta parameters current wording is not explicit enough, suggested update

https://docs.puppetlabs.com/references/latest/function.html#generate

Calls an external command on the Puppet master and returns the results of the command. Any arguments are passed to the external command as arguments. If the generator does not exit with return code of 0, the generator is considered to have failed and a parse error is thrown. Generators can only have file separators, alphanumerics, dashes, and periods in them. This function will attempt to protect you from malicious generator calls (e.g., those with ‘..’ in them), but it can never be entirely safe. No subshell is used to execute generators, so all shell metacharacters are passed directly to the generator, and all metacharacters are returned by the function, consider cleaning white space from any string generated.