s3: make --s3-v2-auth flag

This is an alternative to setting the region to "other-v2-signature"
which is inconvenient for multi-region providers.

docs/content/s3.md

@@ -894,6 +894,20 @@ Some providers (eg Aliyun OSS or Netease COS) require this set to false.
 - Type:        bool
 - Default:     true
 
+#### --s3-v2-auth
+
+If true use v2 authentication.
+
+If this is false (the default) then rclone will use v4 authentication.
+If it is set then rclone will use v2 authentication.
+
+Use this only if v4 signatures don't work, eg pre Jewel/v10 CEPH.
+
+- Config:      v2_auth
+- Env Var:     RCLONE_S3_V2_AUTH
+- Type:        bool
+- Default:     false
+
 <!--- autogenerated options stop -->
 
 ### Anonymous access to public buckets ###