[PA] glossary: Add some extra terms

Bartek notes in his talk that "partition," "root," and "allocator"
usually mean the same thing in PA code. This CL adds that note to
the glossary; in addition, it surfaces the definition of "extent"
from `partition_page.h`.

Change-Id: Ic6c4eac38b9c416d87281149d3f8c80894eb9e49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3788929
Auto-Submit: Kalvin Lee <kdlee@chromium.org>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/main@{#1029587}