[scudo] Update documentation with Scudo's "permissive" mode.

[flowerhack]

Summary

The current docs describe all of Scudo's public-facing options, but it's
a little unclear whether the quarantine is on/off my default, and what
the best settings are if you just want to "try out" Scudo in the least
intrusive way.

I thought describing such settings would be helpful so I added a section
for this "permissive" mode.  Let me know if y'all think it'd be useful
too!

[cryptoad]

You probably can add as well the couple others that default to true that could be set to false as well:

• DeallocationTypeMismatch
• DeleteSizeMismatch

[flowerhack]

added the extra options!

llvm/docs/ScudoHardenedAllocator.rst

 | ZeroContents                | false          | false          | Whether or not we zero chunk contents on       |
 |                             |                |                | allocation and deallocation.                   |
 +-----------------------------+----------------+----------------+------------------------------------------------+
 
 Allocator related common Sanitizer options can also be passed through Scudo
 options, such as ``allocator_may_return_null`` or ``abort_on_error``. A detailed
 list including those can be found here:
 https://github.com/google/sanitizers/wiki/SanitizerCommonFlags.
+
+"Permissive" Scudo
+------------------
+
+It may be useful to *first* try using Scudo in its most permissive, most
+performant mode, and then remove/add options to make Scudo more secure, based
+on your application's needs.
+
+The recommended settings for such usage are as follows:
+
+.. code::
+
+  QuarantineSizeKb=0;ThreadLocalQuarantineSizeKb=0;abort_on_error=1
+
+The first two settings completely disable the quarantine.
+
+`abort_on_error` has Scudo print stack traces when it encounters a memory
+error, which is useful for uncovering the root cause of such errors, if your
+application encounters them frequently, but is not recommended for production
+use.