Add doc for used_memory_peak_time per yzc-yzc

Author: dwdougherty

content/commands/info.md

@@ -152,6 +152,7 @@ Here is the meaning of all fields in the **memory** section:
 *   `used_memory_rss_human`: Human readable representation of previous value
 *   `used_memory_peak`: Peak memory consumed by Redis (in bytes)
 *   `used_memory_peak_human`: Human readable representation of previous value
+*   `used_memory_peak_time`: Time when peak memory was recorded
 *   `used_memory_peak_perc`: The percentage of `used_memory` out of `used_memory_peak`
 *   `used_memory_overhead`: The sum in bytes of all overheads that the server
      allocated for managing its internal data structures