docs: auto generate backend options documentation

This inserts the output of "rclone help backend xxx" into the help
pages for each backend.

docs/content/b2.md

@@ -294,6 +294,108 @@ server to the nearest millisecond appended to them.
 Note that when using `--b2-versions` no file write operations are
 permitted, so you can't upload files or delete them.
 
-<!--- autogenerated options start - edit in backend/backend.go options  -->
-<!--- autogenerated options stop  -->
+<!--- autogenerated options start - DO NOT EDIT, instead edit fs.RegInfo in backend/b2/b2.go then run make backenddocs -->
+### Standard Options
+
+Here are the standard options specific to b2 (Backblaze B2).
+
+#### --b2-account
+
+Account ID or Application Key ID
+
+- Config:      account
+- Env Var:     RCLONE_B2_ACCOUNT
+- Type:        string
+- Default:     ""
+
+#### --b2-key
+
+Application Key
+
+- Config:      key
+- Env Var:     RCLONE_B2_KEY
+- Type:        string
+- Default:     ""
+
+#### --b2-hard-delete
+
+Permanently delete files on remote removal, otherwise hide files.
+
+- Config:      hard_delete
+- Env Var:     RCLONE_B2_HARD_DELETE
+- Type:        bool
+- Default:     false
+
+### Advanced Options
+
+Here are the advanced options specific to b2 (Backblaze B2).
+
+#### --b2-endpoint
+
+Endpoint for the service.
+Leave blank normally.
+
+- Config:      endpoint
+- Env Var:     RCLONE_B2_ENDPOINT
+- Type:        string
+- Default:     ""
+
+#### --b2-test-mode
+
+A flag string for X-Bz-Test-Mode header for debugging.
+
+This is for debugging purposes only. Setting it to one of the strings
+below will cause b2 to return specific errors:
+
+  * "fail_some_uploads"
+  * "expire_some_account_authorization_tokens"
+  * "force_cap_exceeded"
+
+These will be set in the "X-Bz-Test-Mode" header which is documented
+in the [b2 integrations checklist](https://www.backblaze.com/b2/docs/integration_checklist.html).
+
+- Config:      test_mode
+- Env Var:     RCLONE_B2_TEST_MODE
+- Type:        string
+- Default:     ""
+
+#### --b2-versions
+
+Include old versions in directory listings.
+Note that when using this no file write operations are permitted,
+so you can't upload files or delete them.
+
+- Config:      versions
+- Env Var:     RCLONE_B2_VERSIONS
+- Type:        bool
+- Default:     false
+
+#### --b2-upload-cutoff
+
+Cutoff for switching to chunked upload.
+
+Files above this size will be uploaded in chunks of "--b2-chunk-size".
+
+This value should be set no larger than 4.657GiB (== 5GB).
+
+- Config:      upload_cutoff
+- Env Var:     RCLONE_B2_UPLOAD_CUTOFF
+- Type:        SizeSuffix
+- Default:     190.735M
+
+#### --b2-chunk-size
+
+Upload chunk size. Must fit in memory.
+
+When uploading large files, chunk the file into this size.  Note that
+these chunks are buffered in memory and there might a maximum of
+"--transfers" chunks in progress at once.  5,000,000 Bytes is the
+minimim size.
+
+- Config:      chunk_size
+- Env Var:     RCLONE_B2_CHUNK_SIZE
+- Type:        SizeSuffix
+- Default:     96M
+
+<!--- autogenerated options stop -->