docs: auto generate backend options documentation

This inserts the output of "rclone help backend xxx" into the help
pages for each backend.

docs/content/crypt.md

@@ -294,8 +294,93 @@ Note that you should use the `rclone cryptcheck` command to check the
 integrity of a crypted remote instead of `rclone check` which can't
 check the checksums properly.
 
-<!--- autogenerated options start - edit in backend/backend.go options  -->
-<!--- autogenerated options stop  -->
+<!--- autogenerated options start - DO NOT EDIT, instead edit fs.RegInfo in backend/crypt/crypt.go then run make backenddocs -->
+### Standard Options
+
+Here are the standard options specific to crypt (Encrypt/Decrypt a remote).
+
+#### --crypt-remote
+
+Remote to encrypt/decrypt.
+Normally should contain a ':' and a path, eg "myremote:path/to/dir",
+"myremote:bucket" or maybe "myremote:" (not recommended).
+
+- Config:      remote
+- Env Var:     RCLONE_CRYPT_REMOTE
+- Type:        string
+- Default:     ""
+
+#### --crypt-filename-encryption
+
+How to encrypt the filenames.
+
+- Config:      filename_encryption
+- Env Var:     RCLONE_CRYPT_FILENAME_ENCRYPTION
+- Type:        string
+- Default:     "standard"
+- Examples:
+    - "off"
+        - Don't encrypt the file names.  Adds a ".bin" extension only.
+    - "standard"
+        - Encrypt the filenames see the docs for the details.
+    - "obfuscate"
+        - Very simple filename obfuscation.
+
+#### --crypt-directory-name-encryption
+
+Option to either encrypt directory names or leave them intact.
+
+- Config:      directory_name_encryption
+- Env Var:     RCLONE_CRYPT_DIRECTORY_NAME_ENCRYPTION
+- Type:        bool
+- Default:     true
+- Examples:
+    - "true"
+        - Encrypt directory names.
+    - "false"
+        - Don't encrypt directory names, leave them intact.
+
+#### --crypt-password
+
+Password or pass phrase for encryption.
+
+- Config:      password
+- Env Var:     RCLONE_CRYPT_PASSWORD
+- Type:        string
+- Default:     ""
+
+#### --crypt-password2
+
+Password or pass phrase for salt. Optional but recommended.
+Should be different to the previous password.
+
+- Config:      password2
+- Env Var:     RCLONE_CRYPT_PASSWORD2
+- Type:        string
+- Default:     ""
+
+### Advanced Options
+
+Here are the advanced options specific to crypt (Encrypt/Decrypt a remote).
+
+#### --crypt-show-mapping
+
+For all files listed show how the names encrypt.
+
+If this flag is set then for each file that the remote is asked to
+list, it will log (at level INFO) a line stating the decrypted file
+name and the encrypted file name.
+
+This is so you can work out which encrypted names are which decrypted
+names just in case you need to do something with the encrypted file
+names, or for debugging purposes.
+
+- Config:      show_mapping
+- Env Var:     RCLONE_CRYPT_SHOW_MAPPING
+- Type:        bool
+- Default:     false
+
+<!--- autogenerated options stop -->
 
 ## Backing up a crypted remote ##