docs: add s += t/s *= n Operation

[zhangenming]

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📚 Documentation preview 📚: https://cpython-previews--142611.org.readthedocs.build/

[skirpichev]

I don't think the second form is an equivalent of n * s. Ditto for concatenation.

Rather you could extend the footnote to mention that some sequence types have support for in-place concatenation (or repetition).

CC @StanFromIreland

[hauntsaninja]

I don't think these need to be documented specifically here. The place where they do need to be documented specifically is for mutable sequence types, and we already do this.

[picnixz]

Those forms are not equivalent because they combine two thing: the operation and the re-assignment. in particular, when writing s *= n, we're modifing s in place. In addition (6) says that "Concatenating immutable sequences always results in a new object" and thus the note is no more correct.

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[picnixz]

I don't think these need to be documented specifically here. The place where they do need to be documented specifically is for mutable sequence types, and we already do this.

I missed that comment. Considering this comment and mine, I'm going to close this. Sorry.