docs: examples of using env(name) input

[nick-youngblut]

New feature

process-reference-inputs states the following for env( name ):

Declare an environment variable input. The received value should be a string, and it will be exported to the task environment as an environment variable given by name.

So, at least to me, that means the following is how it would be implemented:

workflow{
    MY_PROC("test_value")
}

process MY_PROC {
    input:
    env MY_ENV_VAR

    output:
    path "test"

    script:
    """
    echo $MY_ENV_VAR && touch test
    """
}

...but that just results in:

ERROR ~ Error executing process > 'MY_PROC'

Caused by:
  No such variable: MY_ENV_VAR -- Check script 'main.nf' at line: 23


Source block:
  """
  echo $MY_ENV_VAR && touch test
  """

It would be helpful to have >=1 example or at least more clarification in the docs on how to use env

[bentsherman]

The process input/output types are documented here with more examples. The reference page is still new and relatively sparse. I haven't decided on what else should be moved into the reference docs vs the main process page which is more of a user guide.