Add comment to File copy on linux/android.

runtime/bin/file_linux.cc

Index: runtime/bin/file_linux.cc
diff --git a/runtime/bin/file_linux.cc b/runtime/bin/file_linux.cc
index 3114d01c0be8aba4161a2881c513190526393cc0..856ec0bd1dd14b61c4b69defceedc5cd7bd43fac 100644
--- a/runtime/bin/file_linux.cc
+++ b/runtime/bin/file_linux.cc
@@ -245,6 +245,9 @@ bool File::Copy(const char* old_path, const char* new_path) {
       result = TEMP_FAILURE_RETRY(
           sendfile64(new_fd, old_fd, &offset, kMaxUint32));
     }
+    // From sendfile man pages:
+    //   Applications may wish to fall back to read(2)/write(2) in the case
+    //   where sendfile() fails with EINVAL or ENOSYS.
     if (result < 0 && (errno == EINVAL || errno == ENOSYS)) {
       const intptr_t kBufferSize = 8 * 1024;
       uint8_t buffer[kBufferSize];