Update instructions for rr and MADV_WIPEONFORK

Bug: 1082304
Change-Id: Ie8acbf67b4304dc67f7c8883b2a418c383b046d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216596
Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771964}

docs/linux/debugging.md

@@ -258,9 +258,12 @@ can also step or execute backwards. This works by first recording a trace
 and then debugging based on that. I recommend installing it by compiling
 [from source](https://github.com/mozilla/rr/wiki/Building-And-Installing).
 
-Currently you must apply a patch adding
-[support for recording the MADV_WIPEONFORK](https://bugs.chromium.org/p/chromium/issues/detail?id=1082304#c6)
-syscall as upstream rr triggers an internal assert on this call.
+As of May 2020, you must build from source for [`MADV_WIPEONFORK`
+support](https://bugs.chromium.org/p/chromium/issues/detail?id=1082304). If you
+get the following error, rr is too old:
+```
+Expected EINVAL for 'madvise' but got result 0 (errno SUCCESS); unknown madvise(18)
+```
 
 Once installed, you can use it like this:
 ```