Update docs to include time and title in arguments with --edit

[pconrad-fb]

Fixes #1072.

The issue was that it wasn't clear how to specify a time when using an external editor.
I added a few notes based on the comment by @micahellison. Ultimately, it might make
sense to describe somewhere the fact that jrnl uses the first line to figure out the time/date,
no matter how you slice it, so that it's clear that you can just start typing Last week: or
whatever, even when you're in an external editor. For now, though, I think these added lines
in the external-editors.md file will help.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed. N/A I think

[micahellison]

Thanks for the PR! I think this is a good spot for it, and each instruction flows into the next really nicely.