docs(core): clarify that signed integers use two's complement

[AudaciousAxiom]

Description

This attempts to clarify in the std documentation how signed integers (i8–i128 and isize) are encoded. I do not mean this to introduce a new stability guarantee, but only to document an already obvious (?) property, which is already documented elsewhere (mostly).

Motivation

My understanding is that signed integers are already guaranteed to be encoded in two's complement. The Reference does state this for i8–i128 (but technically does not for isize, at least not in the same place), and in documenting numeric casts and negation operators:

Remember that signed integers are always represented using two’s complement.

This therefore does not seem architecture or platform-dependent. However it is also not specified explicitly in the FLS.

In addition, I could not find this in the std documentation, while this seems required to understand the behavior of shr operations and of as casts in particular.

If this is something you think is worth documenting explicitly, the exact phrasing can likely be refined (it is currently modeled on floats'). If not, feel free to close this PR.

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