gh-143674: document complex representation using f/d formats in struct

[darshan2456]

This PR documents the existing convention for representing complex numbers
using scalar floating-point format characters in the struct module.

Specifically, it clarifies that complex numbers may be represented as two
consecutive floating-point values (real part followed by imaginary part),
with the f and d format characters determining the component type.

The clarification is applied consistently to both the runtime docstring
and the library documentation.

• Issue: Struct module docstring lacks format characters for complex types (F/D) #143674

---

📚 Documentation preview 📚: https://cpython-previews--143879.org.readthedocs.build/

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[darshan2456]

This is a documentation-only change; no NEWS entry is needed. Could a triager please add the skip news label?