gh-131885: Document / for codecs functions

[donbarbos]

1. I found that CodecInfo class signature contains an undocumented (non-public) _is_text_encoding argument:

cpython/Lib/codecs.py

Lines 94 to 96 in c2ac662

	def__new__(cls, encode, decode, streamreader=None, streamwriter=None,
	incrementalencoder=None, incrementaldecoder=None, name=None,
	*, _is_text_encoding=None):

Signature, inspect.signature(codecs.CodecInfo):

<Signature (encode, decode, streamreader=None, streamwriter=None, incrementalencoder=None, incrementaldecoder=None, name=None, *, _is_text_encoding=None)> 

I don't think this should be documented, but I thought I'd point it out just in case.

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2. I'm not a fan of using / and I also want to point out that it doesn't seem natural for codecs functions to use positional-only arguments, since they are only used in the functions that I updated. I would suggest getting rid of / in the code instead of documenting it.

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📚 Documentation preview 📚: https://cpython-previews--131992.org.readthedocs.build/

• Issue: Update function signatures to use * and / as needed #131885

[vstinner]

LGTM

[python-cla-bot]

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[vstinner]

Merged, thank you.