[3.11] [pprint]: Add docstring about `PrettyPrinter.underscore_number… #5

Job	Run time
Check for source changes	9s
Address sanitizer	8m 48s
Ubuntu SSL tests with OpenSSL (1.1.1w)	3m 0s
Check if the ABI has changed	1m 25s
Check if generated files are up to date	3m 8s
Ubuntu SSL tests with OpenSSL (3.0.11)	3m 7s
Ubuntu SSL tests with OpenSSL (3.1.3)	3m 15s
Docs	0s
macOS / build and test	11m 44s
Ubuntu / build and test	8m 16s
Windows / build and test (x64)	14m 26s
Windows / build (arm64)	2m 52s
Windows / build and test (x86)	13m 20s
All required checks pass	1s
	1h 13m 31s

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