Firefox only returns some errors as downloads.InterruptReason (#25655)

* downloads.InterruptReason support limitations in Firefox

* Note rewording and formatting

* Close note with period

Author: rebloor

webextensions/api/downloads.json

@@ -1230,7 +1230,8 @@
               },
               "edge": "mirror",
               "firefox": {
-                "version_added": "47"
+                "version_added": "47",
+                "notes": "Only returns these errors: `NETWORK_FAILED`, `FILE_FAILED`, `CRASH`, `USER_CANCELED`, `SERVER_BAD_CONTENT`, `SERVER_FORBIDDEN`, `SERVER_UNAUTHORIZED`, and `SERVER_FAILED`."
               },
               "firefox_android": {
                 "version_added": "48",