System.Int64 improper 32-bit heading and some general commentary/suggestions for Int32 and Int64

### Describe the issue or suggestion

[System.Int64](https://github.com/dotnet/docs/blob/71d13776de3e2bce47bbc71978fc9f8e37acd3e8/docs/fundamentals/runtime-libraries/system-int64.md?plain=1#L74) has a reference to 32-bit that looks like it got overlooked when all that extra documentation was added.

Also, a commentary on terminology for clarity:

I think it would be best to avoid using the word `decimal` in those articles, in particular, when talking about base-10 numbers, as it could potentially be confused with the `decimal` type, discussed in the same section of the documentation.

_Originally posted by @dodexahedron in https://github.com/dotnet/docs/issues/24287#issuecomment-2062872072_