[components/libc/posix]add comments for rwlock APIs. #4411

Job	Run time
RT-Thread Online Packages (STM32F407 RT-Spark)	6m 59s
RTduino/Arduino Libraries (STM32F412 Nucleo)	14m 26s
RTduino/Arduino Libraries (Raspberry Pico)	13m 4s
others_at32_hc32_ht32	6m 39s
stm32l4_f0_f1	15m 0s
stm32_f2_f4	9m 58s
stm32_f7_g0_h7_mp15_u5_h5_wb5	6m 21s
nxp_renesas	5m 36s
nuvoton	5m 24s
gd32_n32_apm32	5m 23s
Infineon_TI_microchip	7m 24s
mips	2m 31s
aarch64	1h 13m 3s
riscv-none	2m 53s
riscv64-unknown	4m 30s
hpmicro	3m 1s
llvm-arm	5m 36s
simulator	3m 53s
ESP32C3	20m 43s
i386-unknown	3m 56s
nordic(yml)	10m 51s
	3h 47m 11s

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