Update GWP-ASan documentation

- Mention its backronym
- Note that it is now enabled by default on macOS/Windows
- List 2 more limitations I thought of

Change-Id: I19108d66b8f3ec88732f62a10fe9777d675139d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879790
Auto-Submit: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709185}

docs/gwp_asan.md

@@ -5,6 +5,9 @@ samples allocations to a debug allocator, similar to ElectricFence or Page Heap,
 causing memory errors to crash and report additional debugging context about
 the error.
 
+It is also known by its recursive backronym, GWP-ASan Will Provide Allocation
+Sanity.
+
 ## Allocator
 
 The GuardedPageAllocator returns allocations on pages buffered on both sides by
@@ -47,10 +50,9 @@ validate the allocator internals before reasoning about them.
 
 ## Status
 
-GWP-ASan is implemented for malloc and PartitionAlloc, but not for Oilpan or v8,
-on Windows and macOS. It is currently enabled by default for malloc. The
-allocator parameters can be manually modified by using an invocation like the
-following:
+GWP-ASan is implemented for malloc and PartitionAlloc. It is enabled by default
+on Windows and macOS. The allocator parameters can be manually modified by using
+an invocation like the following:
 
 ```shell
 chrome --enable-features="GwpAsanMalloc<Study" \
@@ -81,6 +83,10 @@ default.
 - GWP-ASan does not hook PDFium's fork of PartitionAlloc.
 - Right-aligned allocations to catch overflows are not perfectly right-aligned,
   so small out-of-bounds accesses may be missed.
+- GWP-ASan does not sample some early allocations that occur before field trial
+  initialization.
+- Depending on the platform, GWP-ASan may or may not hook malloc allocations
+  that occur in code not linked directly against Chrome.
 
 ## Testing