[IdentityManager] Add documentation of IdentityManager internals

This CL adds documentation of how the various core internal classes
support the public IdentityManager API surface. The target audience is
engineers looking to maintain/change/enhance the implementation of
IdentityManager (e.g., members of the signin team).

Bug: 970295
Change-Id: I73c13120ec38c791b8b2f973c026c8fca47148d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722935
Reviewed-by: Lowell Manners <lowell@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682237}