Add documentation about how the editor must be a blocking process

[micahellison]

Documentation Change

Affected Page(s)

• https://jrnl.sh/en/stable/external-editors/
• https://jrnl.sh/en/stable/reference-config-file/

What Could Be Better?

When I saw #1451 about getting neovide to work in jrnl, I realized that it wasn't clear in the docs that the editor needs to be a blocking process, rather than returning control to jrnl immediately.

• External Editors page

  • Before going into all the examples on how to make various editors into blocking processes, it should start by mentioning that this is required for any editor.
  • May be worth mentioning that many editors are blocking by default, such as vim.
  • The failure scenario for editing jrnl without a blocking editor is that jrnl just finishes running before you edit the entry. Mention that too, so people know what to look for.

• Config reference page

  • It states that "jrnl processes the file once the editor is closed" which is only true if the editor is a blocking process. The text should change a bit to reflect that.