[docs] Explain download progress event when no Content-Length speci…

…fied (#7530)

* [docs] Explain download progress event when no `Content-Length` specified

* [docs] Add missing header explanation

docs/pages/versions/unversioned/sdk/filesystem.md

@@ -304,7 +304,7 @@ Create a `DownloadResumable` object which can start, pause, and resume a downloa
   This function is called on each data write to update the download progress. An object with the following fields are passed:
 
   - **totalBytesWritten (_number_)** -- The total bytes written by the download operation.
-  - **totalBytesExpectedToWrite (_number_)** -- The total bytes expected to be written by the download operation.
+  - **totalBytesExpectedToWrite (_number_)** -- The total bytes expected to be written by the download operation. A value of `-1` means that the server did not return the `Content-Length` header and the total size is unknown. Without this header, you won't be able to track the download progress.
 
 - **resumeData (_string_)** -- The string which allows the api to resume a paused download. This is set on the `DownloadResumable` object automatically when a download is paused. When initializing a new `DownloadResumable` this should be `null`.
 

docs/pages/versions/v36.0.0/sdk/filesystem.md

@@ -304,7 +304,7 @@ Create a `DownloadResumable` object which can start, pause, and resume a downloa
   This function is called on each data write to update the download progress. An object with the following fields are passed:
 
   - **totalBytesWritten (_number_)** -- The total bytes written by the download operation.
-  - **totalBytesExpectedToWrite (_number_)** -- The total bytes expected to be written by the download operation.
+  - **totalBytesExpectedToWrite (_number_)** -- The total bytes expected to be written by the download operation. A value of `-1` means that the server did not return the `Content-Length` header and the total size is unknown.
 
 - **resumeData (_string_)** -- The string which allows the api to resume a paused download. This is set on the `DownloadResumable` object automatically when a download is paused. When initializing a new `DownloadResumable` this should be `null`.
 

docs/pages/versions/v37.0.0/sdk/filesystem.md

@@ -304,7 +304,7 @@ Create a `DownloadResumable` object which can start, pause, and resume a downloa
   This function is called on each data write to update the download progress. An object with the following fields are passed:
 
   - **totalBytesWritten (_number_)** -- The total bytes written by the download operation.
-  - **totalBytesExpectedToWrite (_number_)** -- The total bytes expected to be written by the download operation.
+  - **totalBytesExpectedToWrite (_number_)** -- The total bytes expected to be written by the download operation. A value of `-1` means that the server did not return the `Content-Length` header and the total size is unknown. Without this header, you won't be able to track the download progress.
 
 - **resumeData (_string_)** -- The string which allows the api to resume a paused download. This is set on the `DownloadResumable` object automatically when a download is paused. When initializing a new `DownloadResumable` this should be `null`.