s3: add --s3-no-head to reducing costs docs - Fixes #2163

docs/content/s3.md

@@ -342,6 +342,16 @@ You'd then do a full `rclone sync` less often.
 
 Note that `--fast-list` isn't required in the top-up sync.
 
+#### Avoiding HEAD requests after PUT
+
+By default rclone will HEAD every object it uploads. It does this to
+check the object got uploaded correctly.
+
+You can disable this with the [--s3-no-head](#s3-no-head) option - see
+there for more details.
+
+Setting this flag increases the chance for undetected upload failures.
+
 ### Hashes ###
 
 For small objects which weren't uploaded as multipart uploads (objects