docs: move documentation for options from docs/content into backends

In the following commit, the documentation will be autogenerated.

backend/dropbox/dropbox.go

@@ -120,8 +120,15 @@ func init() {
 			Name: config.ConfigClientSecret,
 			Help: "Dropbox App Client Secret\nLeave blank normally.",
 		}, {
-			Name:     "chunk_size",
-			Help:     fmt.Sprintf("Upload chunk size. Max %v.", fs.SizeSuffix(maxChunkSize)),
+			Name: "chunk_size",
+			Help: fmt.Sprintf(`Upload chunk size. (< %v).
+
+Any files larger than this will be uploaded in chunks of this size.
+
+Note that chunks are buffered in memory (one at a time) so rclone can
+deal with retries.  Setting this larger will increase the speed
+slightly (at most 10%% for 128MB in tests) at the cost of using more
+memory.  It can be set smaller if you are tight on memory.`, fs.SizeSuffix(maxChunkSize)),
 			Default:  fs.SizeSuffix(defaultChunkSize),
 			Advanced: true,
 		}},