Add some description for (malloc/calloc/free/realloc)

Author: chansuke

library/std/src/sys/wasm/alloc.rs

@@ -24,24 +24,28 @@ static mut DLMALLOC: dlmalloc::Dlmalloc = dlmalloc::DLMALLOC_INIT;
 unsafe impl GlobalAlloc for System {
     #[inline]
     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        // SAFETY:  DLMALLOC.malloc() is guranteed to be safe since lock::lock() aqcuire a globl lock
         let _lock = lock::lock();
         unsafe { DLMALLOC.malloc(layout.size(), layout.align()) }
     }
 
     #[inline]
     unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
+        // SAFETY:  DLMALLOC.calloc() is guranteed to be safe since lock::lock() aqcuire a globl lock
         let _lock = lock::lock();
         unsafe { DLMALLOC.calloc(layout.size(), layout.align()) }
     }
 
     #[inline]
     unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        // SAFETY:  DLMALLOC.free() is guranteed to be safe since lock::lock() aqcuire a globl lock
         let _lock = lock::lock();
         unsafe { DLMALLOC.free(ptr, layout.size(), layout.align()) }
     }
 
     #[inline]
     unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
+        // SAFETY:  DLMALLOC.realloc() is guranteed to be safe since lock::lock() aqcuire a globl lock
         let _lock = lock::lock();
         unsafe { DLMALLOC.realloc(ptr, layout.size(), layout.align(), new_size) }
     }