rune/libenclave: Supply the missing return value description information for PAL API v2

Signed-off-by: NYilin Li <YiLin.Li@linux.alibaba.com>

rune/libenclave/internal/runtime/pal/spec_v2.md

@@ -18,7 +18,7 @@ N/A
 
 ### Return value
 ```
-N/A
+@int: the PAL API version of the current enclave runtime.
 ```
 
 ## 2.pal_init()
@@ -107,6 +107,13 @@ int pal_exec(struct pal_exec_args *attr);
 @exit_value: The exit value of the process.
 ```
 
+### Return value
+```
+0: Success
+-EINVAL: Invalid argument
+-ENOSYS: The function is not supported
+```
+
 ## 5.pal_kill()
 
 ### Description