Improve `printf`-style formatting docs for non-built-in types

Add a description about how Decimal behaves differently with format and % formatting in https://docs.python.org/3/library/string.html#format-specification-mini-language

For __format__, Decimal is formatted with the highest possible precision as it is.

For % formatting, Decimal is converted to float before formatting.

These behaviors have not changed since they were added.

• The Decimal class was added in Python 2.4 (https://docs.python.org/2.4/whatsnew/whatsnew24.html).
• The __format__ method was added in Python 2.6 (https://docs.python.org/2.6/whatsnew/2.6.html).

Compare:

# ./test.py
import sys
from decimal import Decimal

for expr in (
    "sys.version",

    "'%f' % 12345123451234512345.0",
    "'%f' % Decimal('12345123451234512345.0')",
    "Decimal('12345123451234512345.0').__format__('f')",

    "'%e' % 0.0",
    "'%e' % Decimal('0.0')",
    "Decimal('0.0').__format__('e')",

    "'%.20e' % 12345123451234512345.0",
    "'%.20e' % Decimal('12345123451234512345.0')",
    "Decimal('12345123451234512345.0').__format__('.20e')",

    "'%.20g' % 12345123451234512345.0",
    "'%.20g' % Decimal('12345123451234512345.0')",
    "Decimal('12345123451234512345.0').__format__('.20g')",
):
    print(expr)
    try:
        print(repr(eval(expr)))
    except Exception:
        print('Exception')
    print('')

# ./test_runner.py
from pathlib import Path
from subprocess import run, PIPE

PYTHON_DIR_ROOT = r"C:\python_dir_root"

def print_row(row, file):
    for v in row: print(f"| {v} ", end='', file=file)
    print("|", file=file)

def print_md_table(headers, rows, file):
    print_row(headers, file=file)
    print_row(['---:'] * len(headers), file=file)
    for row in rows: print_row(row, file=file)
    print(file=file)

rows = []
for dir in Path(PYTHON_DIR_ROOT).iterdir():
    out = run([str(dir/"python.exe"), "./test.py"], stdout=PIPE).stdout
    out = out.decode('utf-8').splitlines()
    headers = [f"`{h}`" for h in out[::3]]
    rows.append(out[1::3])

with open("./test_result.md", 'w', encoding='utf-8') as fp:
    print_md_table(headers, rows, file=fp)

Linked PRs

• gh-142117
• gh-142126

Start at the format specification mini-language section of docs.python.org/3/library/string.html and review the Decimal examples and formatting behavior described in the issue. Check linked PRs gh-142117 and gh-142126 first; the work is done when the documentation clearly distinguishes format from % formatting for Decimal.