Add note on BigInteger bin/hex formatting of positive values

[SolalPirelli]

Summary

Fixes dotnet/runtime#115618.

It's somewhat unexpected that "print 3 with 2 binary digits" returns a string of length 3 ("011"), but also makes sense given the round-trip requirement and the historical context of not using - for negative bin/hex numbers.

I think documenting this behavior would be useful.

---

Internal previews

📄 File	🔗 Preview link
docs/standard/base-types/standard-numeric-format-strings.md	Standard numeric format strings