docs(cli): cache hard limits flags (kopia#3846)

site/content/docs/Advanced/Caching/_index.md

@@ -57,6 +57,14 @@ $ kopia cache set --max-list-cache-duration=300s
 Note the cache sizes are not hard limits: cache is swept periodically (every few minutes) to bring
 the total usage below the defined limit by removing least-recently used cache items.
 
+A hard limit can be set if required via the corresponding `limit` flag:
+```
+# set the maximum content cache size to 30GB
+$ kopia cache set --content-cache-size-limit-mb=30000
+# set the maximum metadata cache size to 20GB
+$ kopia cache set --metadata-cache-size-limit-mb=20000
+```
+
 ### Clearing Cache
 
 Cache can be cleared on demand by `kopia cache clear` or by simply removing appropriate files. It is always safe to remove files from cache.