* command/init: return an error with invalid -backend-config files
The -backend-config flag expects a set of key-value pairs or a file
containing key-value pairs. If the file instead contains a full backend
configuration block, it was silently ignored. This commit adds a check
for blocks in the file and returns an error if they are encountered.
Fixes#24845
* emphasize backend configuration file in docs
Kristin Laemmert