Docs: Clarify cache file deletion (fixes #4943) (#6712)

docs/user-guide/command-line-interface.md

@@ -169,7 +169,9 @@ Examples:
 
 #### `--cache`
 
-Store the info about processed files in order to only operate on the changed ones.
+Store the info about processed files in order to only operate on the changed ones. The cache is stored in `.eslintcache` by default. Enabling this option can dramatically improve ESLint's running time by ensuring that only changed files are linted.
+
+**Note:** If you run ESLint with `--cache` and then run ESLint without `--cache`, the `.eslintcache` file will be deleted. This is necessary because the results of the lint might change and make `.eslintcache` invalid. If you want to control when the cache file is deleted, then use `--cache-location` to specify an alternate location for the cache file.
 
 #### `--cache-file`