docs(config): document npm config fix (#5819)

Author: wraithgar

docs/lib/content/commands/npm-config.md

@@ -83,6 +83,16 @@ npm config edit
 Opens the config file in an editor.  Use the `--global` flag to edit the
 global config.
 
+#### fix
+
+```bash
+npm config fix
+```
+
+Attempts to repair invalid configuration items.  Usually this means
+attaching authentication config (i.e. `_auth`, `_authToken`) to the
+configured `registry`.
+
 ### Configuration
 
 <!-- AUTOGENERATED CONFIG DESCRIPTIONS -->