docs: storage.md: clarify storage.tsdb.retention.time description

Signed-off-by: tesla59 <nishant@heim.id>

docs/storage.md

@@ -84,8 +84,10 @@ or 31 days, whichever is smaller.
 Prometheus has several flags that configure local storage. The most important are:
 
 - `--storage.tsdb.path`: Where Prometheus writes its database. Defaults to `data/`.
-- `--storage.tsdb.retention.time`: When to remove old data. Defaults to `15d`.
-  Overrides `storage.tsdb.retention` if this flag is set to anything other than default.
+- `--storage.tsdb.retention.time`: How long to retain samples in storage. When this flag is
+  set it overrides `storage.tsdb.retention`. If neither this flag nor `storage.tsdb.retention`
+  nor `storage.tsdb.retention.size` is set, the retention time defaults to `15d`.
+  Units Supported: y, w, d, h, m, s, ms.
 - `--storage.tsdb.retention.size`: The maximum number of bytes of storage blocks to retain.
   The oldest data will be removed first. Defaults to `0` or disabled. Units supported:
   B, KB, MB, GB, TB, PB, EB. Ex: "512MB". Based on powers-of-2, so 1KB is 1024B. Only