add more explanation to IO-waiting message during precompilation

doc/src/devdocs/precompile_hang.md

@@ -12,8 +12,8 @@ If you follow the advice and hit `Ctrl-C`, you might see
   1 dependency had warnings during precompilation:
 ┌ Test1 [ac89d554-e2ba-40bc-bc5c-de68b658c982]
 │  [pid 2745] waiting for IO to finish:
-│   TYPE[FD/PID]       @UV_HANDLE_T->DATA
-│   timer              @0x55580decd1e0->0x7f94c3a4c340
+│   Handle type        uv_handle_t->data
+│   timer              0x55580decd1e0->0x7f94c3a4c340
 ```
 
 and, depending on how long you waited, this may repeat.

src/jl_uv.c

@@ -51,9 +51,9 @@ static void walk_print_cb(uv_handle_t *h, void *arg)
     npad += strlen(type);
     pad += npad < strlen(pad) ? npad : strlen(pad);
     if (fd == -1)
-        jl_safe_printf(" %s   %s@%p->%p\n", type,             pad, (void*)h, (void*)h->data);
+        jl_safe_printf(" %s   %s%p->%p\n", type,             pad, (void*)h, (void*)h->data);
     else
-        jl_safe_printf(" %s[%zd] %s@%p->%p\n", type, (size_t)fd, pad, (void*)h, (void*)h->data);
+        jl_safe_printf(" %s[%zd] %s%p->%p\n", type, (size_t)fd, pad, (void*)h, (void*)h->data);
 }
 
 static void wait_empty_func(uv_timer_t *t)
@@ -63,9 +63,12 @@ static void wait_empty_func(uv_timer_t *t)
     if (!uv_loop_alive(t->loop))
         return;
     jl_safe_printf("\n[pid %zd] waiting for IO to finish:\n"
-                   " TYPE[FD/PID]       @UV_HANDLE_T->DATA\n",
+                   " Handle type        uv_handle_t->data\n",
                    (size_t)uv_os_getpid());
     uv_walk(jl_io_loop, walk_print_cb, NULL);
+    if (jl_generating_output() && jl_options.incremental) {
+        jl_safe_printf("This means that a package has started a background task or event source that has not finished running. For precompilation to complete successfully, the event source needs to be closed explicitly. See the developer documentation on fixing precompilation hangs for more help.\n");
+    }
     jl_gc_collect(JL_GC_FULL);
 }