[3.14] gh-143674: Document F/D complex format characters in struct module (GH-143675...

[3.14] gh-143674: Document F/D complex format characters in struct module (GH-143675) (#143918)

gh-143674: Document F/D complex format characters in struct module (GH-143675)

Add documentation for the F (complex from two floats) and D (complex
from two doubles) format characters in the struct module docstring.
These format characters were implemented but not documented.
(cherry picked from commit 3e932257980be8907698c378c23026e262ba066c)

Co-authored-by: Muneeb Ullah <moneebullah25@gmail.com>
Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>