bpo-35224: Additional documentation for Assignment Expressions (GH-15935) (GH-15967)

Add or update assignment expression documentation for:
- FAQ - Design
- Reference - Expressions
- Reference - Lexical Analysis

https://bugs.python.org/issue35224

Automerge-Triggered-By: @matrixise
(cherry picked from commit 6357c95716d89ac1f80587fbc4133df8d2e8396c)

Co-authored-by: Emily Morehouse <emily@cuttlesoft.com>
4 files changed