base: Minor improvement to base::SequenceChecker doc comments.

Change-Id: I7ab1de6fcc9b6778ce29e1fdd85d3e926dcfe0d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2610632Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840425}

base/sequence_checker.h

@@ -26,8 +26,8 @@
 //   class MyClass {
 //    public:
 //     MyClass() {
-//       // It's sometimes useful to detach on construction for objects that are
-//       // constructed in one place and forever after used from another
+//       // Detaching on construction is necessary for objects that are
+//       // constructed on one sequence and forever after used from another
 //       // sequence.
 //       DETACH_FROM_SEQUENCE(my_sequence_checker_);
 //     }