bpo-39369 Doc: Update mmap readline method documentation

[Pardim93]

Update mmap readline method description. The fact that the readline method does update the file position should not be ignored since this might give the impression for the programmer that it doesn't update it.

https://bugs.python.org/issue39369

[the-knights-who-say-ni]

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[KenanHArik]

Thanks @Pardim93. At PyCascade Sprint. I have reviewed this update for clarification and recommend this PR to get merged @willingc.

[willingc]

Thanks @KenanHArik for reviewing this PR. Congrats @Pardim93 for your first contribution to CPython 🎉