docs: add more details to -cacheDataPath vmselect flag (#6708)

vmselect will create `./tmp` dir under `cacheDataPath`. If
`cacheDataPath` is set to `/`, vmselect will use `/tmp`.

content under `/tmp` dir might be auto removed based on the OS
behaviour. See:
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5770

- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

---------

Signed-off-by: hagen1778 <roman@victoriametrics.com>
Co-authored-by: hagen1778 <roman@victoriametrics.com>

docs/Cluster-VictoriaMetrics.md

@@ -56,6 +56,8 @@ It increases cluster availability, and simplifies cluster maintenance as well as
 
 ![Cluster Scheme](Cluster-VictoriaMetrics_cluster-scheme.webp)
 
+> Note that `vmselect` despite being stateless still requires some disk space (a few GBs) for temporary caches. Refer to the `-cacheDataPath` command-line flag for more details.
+
 ## Multitenancy
 
 VictoriaMetrics cluster supports multiple isolated tenants (aka namespaces).
@@ -1308,7 +1310,7 @@ Below is the output for `/path/to/vmselect -help`:
   -blockcache.missesBeforeCaching int
      The number of cache misses before putting the block into cache. Higher values may reduce indexdb/dataBlocks cache size at the cost of higher CPU and disk read usage (default 2)
   -cacheDataPath string
-     Path to directory for cache files. By default, the cache is not persisted.
+     Path to directory for cache files and temporary query results. By default, the cache won't be persisted, and temporary query results will be placed under /tmp/searchResults. If set, the cache will be persisted under cacheDataPath/rollupResult, and temporary query results will be placed under cacheDataPath/tmp/searchResults.
   -cacheExpireDuration duration
      Items are removed from in-memory caches after they aren't accessed for this duration. Lower values may reduce memory usage at the cost of higher CPU usage. See also -prevCacheRemovalPercent (default 30m0s)
   -cluster.tls