Updates the folder input text and adds docs note for how to figure out the folder separator value

docs/usage_overview.rst

@@ -178,6 +178,14 @@ These are as follows:
     automatically or manually and tell paperless to move them to yet another folder
     after consumption. It's up to you.
 
+.. note::
+
+    When defining a mail rule with a folder, you may need to try different characters to
+    define how the sub-folders are separated.  Common values include ".", "/" or "|", but
+    this varies by the mail server.  Unfortunately, this isn't a value we can determine
+    automatically.  Either check the documentation for your mail server, or check for
+    errors in the logs and try different folder separator values.
+
 .. note::
 
     Paperless will process the rules in the order defined in the admin page.

src/paperless_mail/models.py

@@ -119,7 +119,10 @@ class MailRule(models.Model):
         _("folder"),
         default="INBOX",
         max_length=256,
-        help_text=_("Subfolders must be separated by dots."),
+        help_text=_(
+            "Subfolders must be separated by a delimiter, often a dot ('.') or "
+            " slash ('/'), but it varies by mail server.",
+        ),
     )
 
     filter_from = models.CharField(