b2 doc fixes

docs/content/b2.md

@@ -104,7 +104,7 @@ method to set the modification time independent of doing an upload.
 ### SHA1 checksums ###
 
 The SHA1 checksums of the files are checked on upload and download and
-will be used in the syncing process. You can use the `--checksum` flag.
+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.
@@ -119,6 +119,11 @@ depending on your hardware, how big the files are, how much you want
 to load your computer, etc.  The default of `--transfers 4` is
 definitely too low for Backblaze B2 though.
 
+Note that uploading big files (bigger than 200 MB by default) will use
+a 96 MB RAM buffer by default.  There can be at most `--transfers` of
+these in use at any moment, so this sets the upper limit on the memory
+used.
+
 ### Versions ###
 
 When rclone uploads a new version of a file it creates a [new version
@@ -185,15 +190,19 @@ system.
 #### --b2-chunk-size valuee=SIZE ####
 
 When uploading large files chunk the file into this size.  Note that
-these chunks are buffered in memory.  100,000,000 Bytes is the minimim
-size (default 96M).
+these chunks are buffered in memory and there might a maximum of
+`--transfers` chunks in progress at once.  100,000,000 Bytes is the
+minimim size (default 96M).
 
 #### --b2-upload-cutoff=SIZE ####
 
-Cutoff for switching to chunked upload (default 4.657GiB ==
-5GB). Files above this size will be uploaded in chunks of
-`--b2-chunk-size`. The default value is the largest file which can be
-uploaded without chunks.
+Cutoff for switching to chunked upload (default 190.735 MiB == 200
+MB). Files above this size will be uploaded in chunks of
+`--b2-chunk-size`.
+
+This value should be set no larger than 4.657GiB (== 5GB) as this is
+the largest file size that can be uploaded.
+
 
 #### --b2-test-mode=FLAG ####