Includes a short description of the ContextVk command APIs.

Also a few small cleanups to other Vulkan back-end documentation.

Bug: angleproject:4029
Change-Id: I3bd04861f27b718e6a94fdae00644c124452bc62
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2070597
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>