[PartitionAlloc] Update the "decommit" documentation

It's a subtle and platform-specific concept. Thanks to wez@ for pointing this
out.

Bug: None
Change-Id: Ia3017e096dd5d92e00cf267cbeb4c2d7977c76c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611335Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841206}

base/allocator/partition_allocator/page_allocator.h

@@ -113,11 +113,11 @@ BASE_EXPORT void SetSystemPagesAccess(
 // http://crbug.com/766882.
 //
 // Decommitted means that physical resources (RAM or swap) backing the allocated
-// virtual address range are released back to the system, but the address space
-// is still allocated to the process (possibly using up page table entries or
-// other accounting resources). Unless PageKeepPermissionsIfPossible disposition
-// is used, any access to a decommitted region of memory is an error and will
-// generate a fault.
+// virtual address range may be released back to the system, but the address
+// space is still allocated to the process (possibly using up page table entries
+// or other accounting resources). There is no guarantee that the pages are
+// zeroed. Unless PageKeepPermissionsIfPossible disposition is used, any access
+// to a decommitted region of memory is an error and will generate a fault.
 //
 // This operation is not atomic on all platforms.
 //