lsblk: add custom output example (#5039)

pages/linux/lsblk.md

@@ -29,3 +29,7 @@
 - Exclude the devices specified by the comma-separated list of major device numbers:
 
 `lsblk -e {{1,7}}`
+
+- Display a customized summary using a comma-separated list of columns:
+
+`lsblk --output {{NAME}},{{SERIAL}},{{MODEL}},{{TRAN}},{{TYPE}},{{SIZE}},{{FSTYPE}},{{MOUNTPOINT}}`