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>
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 73a2f27..d9db33a 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -1784,6 +1784,8 @@
 +-----------------------------------------------+-------------------------------------+
 | Operator                                      | Description                         |
 +===============================================+=====================================+
+| ``:=``                                        | Assignment expression               |
++-----------------------------------------------+-------------------------------------+
 | :keyword:`lambda`                             | Lambda expression                   |
 +-----------------------------------------------+-------------------------------------+
 | :keyword:`if <if_expr>` -- :keyword:`!else`   | Conditional expression              |