[Threading and Tasks] Add Nomenclature section to docs

We get frequent questions about these terms and it's time we document
them explicitly. "thread-compatible" is a new term we haven't used in
the past (we just used initialized-once-read-only-forever-after)
but let's use that from now on as it's more consistent with the
internal docs:
https://engdoc.corp.google.com/eng/doc/devguide/cpp/primer.md?cl=head#thread_safety

[email protected]

Change-Id: Ia20eadfb1910d54524bd0c08eb70dd60d94eae39
Reviewed-on: https://chromium-review.googlesource.com/c/1456642
Auto-Submit: Gabriel Charette <[email protected]>
Commit-Queue: François Doray <[email protected]>
Reviewed-by: François Doray <[email protected]>
Cr-Commit-Position: refs/heads/master@{#629730}
1 file changed