Add note on cache eviction

Signed-off-by: Daniel Carl Jones <[email protected]>

doc/CONFIGURATION.md

@@ -236,6 +236,8 @@ To enable caching, use the `--cache <CACHE_DIR>` command-line flag.
 This flag will enable caching of metadata using a default time-to-live (TTL) of 1 second.
 Object content will also be cached within the cache directory specified.
 Mountpoint will create a directory within this path, removing any files or directories within it at mount time and exit.
+When available space for the cache on disk is limited,
+content of files least recently accessed will be evicted to allow new data to be cached.
 When running multiple Mountpoint processes on the same host,
 you should use unique cache directories to avoid different processes interfering with the others' cache content.