[Telink] Increase LIBC size for tl7218x_retention

- increase COMMON_LIBC_MALLOC_ARENA_SIZE from 16kB to 20kB
for tl7218x_retention to avoid overflow issue.
- improve configs description.

Signed-off-by: Damien Ji <yinghao.ji@telink-semi.com>

Author: damien0x0023

config/telink/chip-module/Kconfig.defaults

@@ -90,7 +90,7 @@ config HEAP_MEM_POOL_SIZE
 config COMMON_LIBC_MALLOC_ARENA_SIZE
     default 20716 if SOC_SERIES_RISCV_TELINK_B9X_RETENTION || \
                      (SOC_RISCV_TELINK_TL321X && ZEPHYR_VERSION_3_3) || \
-                     (SOC_RISCV_TELINK_TL721X && SOC_SERIES_RISCV_TELINK_TLX_RETENTION && ZEPHYR_VERSION_3_3)
+                     (SOC_RISCV_TELINK_TL721X && SOC_SERIES_RISCV_TELINK_TLX_RETENTION)
     default 16384 if SOC_RISCV_TELINK_TL721X || (SOC_RISCV_TELINK_TL321X && !ZEPHYR_VERSION_3_3)
     default 12288
 
@@ -307,12 +307,14 @@ config OPENTHREAD_CSMABACKOFF_OPTIMIZATION
 	bool "Skip the first backoff during sending data request"
 	depends on CHIP_ENABLE_ICD_SUPPORT
 	default n
+	help
+	  Help save idle time during IEEE 802.15.4 communication
 
 config IEEE802154_TLX_OPTIMIZATION
-	bool "optimize the rf performance for tlx"
+	bool "Optimize 802.15.4 RF performance for TLX SoCs"
 	default n
 	help
-	  optimize the rf performance for tlx.
+	  Improve RF performance in IEEE 802.15.4 communication on TLX SoCs.
 
 config OPENTHREAD_THREAD_STACK_SIZE
     default 2400 if PM || SOC_RISCV_TELINK_TL321X