b2: correct docs on SHA1s on large files

docs/content/b2.md

@@ -116,8 +116,22 @@ method to set the modification time independent of doing an upload.
 The SHA1 checksums of the files are checked on upload and download and
 will be used in the syncing process.
 
-Large files which are uploaded in chunks will store their SHA1 on the
-object as `X-Bz-Info-large_file_sha1` as recommended by Backblaze.
+Large files (bigger than the limit in `--b2-upload-cutoff`) which are
+uploaded in chunks will store their SHA1 on the object as
+`X-Bz-Info-large_file_sha1` as recommended by Backblaze.
+
+For a large file to be uploaded with an SHA1 checksum, the source
+needs to support SHA1 checksums. The local disk supports SHA1
+checksums so large file transfers from local disk will have an SHA1.
+See [the overview](/overview/#features) for exactly which remotes
+support SHA1.
+
+Sources which don't support SHA1, in particular `crypt` will upload
+large files without SHA1 checksums.  This may be fixed in the future
+(see [#1767](https://github.com/ncw/rclone/issues/1767)).
+
+Files sizes below `--b2-upload-cutoff` will always have an SHA1
+regardless of the source.
 
 ### Transfers ###
 
@@ -233,15 +247,6 @@ start and finish the upload) and another 2 requests for each chunk:
 /b2api/v1/b2_finish_large_file
 ```
 
-### B2 with crypt ###
-
-When using B2 with `crypt` files are encrypted into a temporary
-location and streamed from there. This is required to calculate the
-encrypted file's checksum before beginning the upload. On Windows the
-%TMPDIR% environment variable is used as the temporary location. If
-the file requires chunking, both the chunking and encryption will take
-place in memory.
-
 ### Specific options ###
 
 Here are the command line options specific to this cloud storage