Mention the redaction process in debugging_with_crash_keys.md.

Also link to debugging_with_crash_keys.md from crash_key.h.

Bug: 1056426
Change-Id: I8652eae061868e0ec220a89071e738fc2fef7ba4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2086388Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746800}

components/crash/core/common/crash_key.h

@@ -19,6 +19,9 @@
 // Annotation interface. Because not all platforms use Crashpad yet, a
 // source-compatible interface is provided on top of the older Breakpad
 // storage mechanism.
+//
+// See https://cs.chromium.org/chromium/src/docs/debugging_with_crash_keys.md
+// for more information on using this.
 #if BUILDFLAG(USE_CRASHPAD_ANNOTATION) || BUILDFLAG(USE_COMBINED_ANNOTATIONS)
 #include "third_party/crashpad/crashpad/client/annotation.h"  // nogncheck
 #endif

docs/debugging_with_crash_keys.md

@@ -32,6 +32,14 @@ things that happen in exceptional cases: the URL of the webpage, command line
 switches, active extension IDs, GPU vendor information, experiment/variations
 information, etc.
 
+## Redaction
+
+Beware that certain on certain platforms (e.g. Android Webview) we
+[sanitize the stack in the dump](https://cs.chromium.org/chromium/src/third_party/crashpad/crashpad/snapshot/sanitized/memory_snapshot_sanitized.h)
+and only crash keys on a
+[whitelist](https://cs.chromium.org/chromium/src/android_webview/common/crash_reporter/crash_keys.cc)
+will be captured.
+
 ## Getting Started with a Single Key-Value Pair
 
 Imagine you are investigating a crash, and you want to know the value of some