fs: document why isPerformingIO is required

lib/internal/fs/streams.js

@@ -255,6 +255,12 @@ ReadStream.prototype._read = function(n) {
 };
 
 ReadStream.prototype._destroy = function(err, cb) {
+  // Usually for async io it is safe to close a file descriptor
+  // even when there are pending operations. However, due to platform
+  // differences file IO is implemented using synchronous operations
+  // running in a thread pool. Therefore, file descriptors are not safe
+  // to close while used in a pending read or write operation. Wait for
+  // any pending IO (kIsPerformingIO) to complete (kIoDone).
   if (this[kIsPerformingIO]) {
     this.once(kIoDone, (er) => close(this, err || er, cb));
   } else {
@@ -416,12 +422,19 @@ WriteStream.prototype._writev = function(data, cb) {
 };
 
 WriteStream.prototype._destroy = function(err, cb) {
+  // Usually for async io it is safe to close a file descriptor
+  // even when there are pending operations. However, due to platform
+  // differences file IO is implemented using synchronous operations
+  // running in a thread pool. Therefore, file descriptors are not safe
+  // to close while used in a pending read or write operation. Wait for
+  // any pending IO (kIsPerformingIO) to complete (kIoDone).
   if (this[kIsPerformingIO]) {
     this.once(kIoDone, (er) => close(this, err || er, cb));
   } else {
     close(this, err, cb);
   }
 };
+
 WriteStream.prototype.close = function(cb) {
   if (cb) {
     if (this.closed) {