Docs: Receipt export explanations

[Lynn Floyd]

In Exporting Templates, it's not clear that when you export, you're exporting all print templates in one file. https://docs.evergreen-ils.org/eg/docs/latest/admin/receipt_template_editor.html

[Gina Monti (gmonti90)]

Submitted a pull request for this.

[Jane Sandberg (sandbergja)]

Thanks for the report, Lynn, and the fix, Gina! Definitely something that has tripped me up before, so I'm very glad to have it in our docs.

I have pushed Gina's fix to master, rel_3_8, and rel_3_7.