fix: clarify reclaim condition on redpanda_memory_allocated_memory

Add "if needed" so the description of the batch cache reclaim behavior
is technically accurate. Follow-up to PR #1656 addressing post-merge
review nit from @bharathv:
#1656 (comment)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: micheleRP

modules/reference/pages/public-metrics-reference.adoc

@@ -889,7 +889,7 @@ endif::[]
 
 === redpanda_memory_allocated_memory
 
-Total memory allocated (in bytes) per CPU shard. This includes all memory currently held by Redpanda on that shard, including memory in the batch cache that has been allocated but could be reclaimed.
+Total memory allocated (in bytes) per CPU shard. This includes all memory currently held by Redpanda on that shard, including memory in the batch cache that has been allocated but could be reclaimed if needed.
 
 *Type*: gauge