Add section about alternatives to docs/danging_ptr_guide.md

Explain that the alternative dangling ptr detector may provide
additional insight when stumped.

Change-Id: I61edfe5e6e5121af9595239e4f2a0633eaf51b79
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4651516
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1164187}

docs/dangling_ptr_guide.md

@@ -22,6 +22,7 @@ See also the general instructions about the dangling pointer detector:
     - [Object vended from C API](#object-vended-from-c-api)
     - [Object conditionally owned](#object-conditionally-owned)
     - [Fallback solution](#fallback-solution-1)
+- [I can't figure out which pointer is dangling](I-can_t-figure-out-which-pointer-is-dangling)
 - [FAQ - Why dangling pointers matter](#faq-why-dangling-pointers-matter)
 
 ## What to do about dangling pointers
@@ -198,6 +199,15 @@ the dangling raw_ptr.
 |--|--|
 |`ExternalAPIDelete(ptr_);`|`ExternalAPIDelete(ptr_.ExtractAsDangling());`|
 
+## I can't figure out which pointer is dangling
+
+Usually this is a matter of straightforward reasoning, but should all else
+fail, another option is to re-build with the alternative dangling pointer
+detector as described in
+[docs/dangling_ptr.md](./dangling_ptr.md#alternative-dangling-pointer-detector-experimental).
+This will show the stacks for object creation, object destruction, and the
+destruction of the object containing the dangling ptr member.
+
 ## FAQ - Why dangling pointers matter
 
 Q. Gee, this is a raw pointer. Does destroying it actually do anything?