billing: Update CustomerPlan.invoiced_through documentation.

Clarify what the invoiced_through field on a CustomerPlan indicates
after the changes to the billing state machine as ledger entries
are processed in invoice_plan.

Updates the plan activity page where this field is shown to support
admin users for these changes as well.

Follow-up to #34643.

corporate/models/plans.py

@@ -120,11 +120,12 @@ class CustomerPlan(AbstractCustomerPlan):
     # an email only once and not every time when cron run.
     reminder_to_review_plan_email_sent = models.BooleanField(default=False)
 
-    # On next_invoice_date, we go through ledger entries that were
-    # created after invoiced_through and process them by generating
-    # invoices for any additional users and/or plan renewal. Once the
-    # invoice is generated, we update the value of invoiced_through
-    # and set it to the last ledger entry we processed.
+    # On next_invoice_date, we call invoice_plan, which goes through
+    # ledger entries that were created after invoiced_through and
+    # process them. An invoice will be generated for any additional
+    # users and/or plan renewal (if it's the end of the billing cycle).
+    # Once all new ledger entries have been processed, invoiced_through
+    # will be have been set to the last ledger entry we checked.
     invoiced_through = models.ForeignKey(
         "LicenseLedger", null=True, on_delete=CASCADE, related_name="+"
     )

corporate/views/plan_activity.py

@@ -56,7 +56,7 @@ def get_plan_ledger(request: HttpRequest, plan_id: int) -> HttpResponse:
     if plan.invoiced_through is not None:
         header_entries.append(
             ActivityHeaderEntry(
-                name="Entry for last invoice",
+                name="Entry last checked during invoicing",
                 value=str(plan.invoiced_through),
             )
         )