Bug 1626804 - Clarify about:memory's `explicit` description. r=erahm

This commit clarifies that only some OS-level allocations are covered; the old
wording made it sound like they all were.

It also moves the allocator-level allocations first in the list, because
they're more common.

Depends on D43842

Differential Revision: https://phabricator.services.mozilla.com/D69287

--HG--
extra : moz-landing-system : lando

toolkit/components/aboutmemory/content/aboutMemory.js

@@ -268,13 +268,14 @@ function newElement(aTagName, aClassName) {
 const explicitTreeDescription =
   "This tree covers explicit memory allocations by the application.  It includes \
 \n\n\
-* allocations made at the operating system level (via calls to functions such as \
-VirtualAlloc, vm_allocate, and mmap), \
-\n\n\
-* allocations made at the heap allocation level (via functions such as malloc, \
+* all allocations made at the heap allocation level (via functions such as malloc, \
 calloc, realloc, memalign, operator new, and operator new[]) that have not been \
 explicitly decommitted (i.e. evicted from memory and swap), and \
 \n\n\
+* some allocations (those covered by memory reporters) made at the operating \
+system level (via calls to functions such as VirtualAlloc, vm_allocate, and \
+mmap), \
+\n\n\
 * where possible, the overhead of the heap allocator itself.\
 \n\n\
 It excludes memory that is mapped implicitly such as code and data segments, \