dlmalloc: add missing docs

vlib/dlmalloc/dlmalloc.v

@@ -238,6 +238,7 @@ mut:
 	release_checks usize
 }
 
+// new creates a new instance of `Dlmalloc` with the given system allocator.
 pub fn new(system_allocator Allocator) Dlmalloc {
 	return Dlmalloc{
 		smallmap:         0
@@ -914,6 +915,7 @@ fn (mut dl Dlmalloc) treemap_is_marked(idx u32) bool {
 	return dl.treemap & (1 << idx) != 0
 }
 
+// malloc allocates a block of memory of the given size.
 pub fn (mut dl Dlmalloc) malloc(size usize) voidptr {
 	unsafe {
 		p := dl.malloc_real(size)

vlib/dlmalloc/dlmalloc_sys_nix.c.v

@@ -112,6 +112,7 @@ fn system_page_size(_ voidptr) usize {
 	return 4096
 }
 
+// get_system_allocator returns an allocator that uses the system allocator.
 pub fn get_system_allocator() Allocator {
 	return Allocator{
 		alloc:            system_alloc

vlib/dlmalloc/dlmalloc_sys_windows.c.v

@@ -28,6 +28,7 @@ fn system_page_size(_ voidptr) usize {
 	return 4096
 }
 
+// get_system_allocator returns an allocator that uses the system allocator.
 pub fn get_system_allocator() Allocator {
 	return Allocator{
 		alloc:            system_alloc