JOB_ID_SEPARATOR should be documented

[bdice]

Issue Description

This pull request adds a feature to Environment classes: by defining an attribute JOB_ID_SEPARATOR, submitted jobs will use that separator string instead of a slash '/' in the submitted job names. This is needed for clusters that do not support slash as a character in the job name, as documented in this issue.

Proposed change (optional)

Add a short note about JOB_ID_SEPARATOR into environments.rst: https://github.com/glotzerlab/signac-docs/blob/master/docs/source/environments.rst

[csadorf]

Why is this not a class attribute with a default value? Then one could document it with a docstring?

[bdice]

Why is this not a class attribute with a default value? Then one could document it with a docstring?

Sounds like a good idea to me. I like that option better than my original proposal. I’ll rename the issue title to be solution-agnostic.