Unclear description regarding equality of "K" and "zzz" time zone format specifier

### Describe the issue or suggestion

The [documentation](https://learn.microsoft.com/en-us/dotnet/standard/base-types/custom-date-and-time-format-strings#KSpecifier) states:

>For DateTimeOffset values, the "K" format specifier is equivalent to the "zzz" format specifier [...]

while this may be the case for serialisation, this behaviour differs when deserializing time strings as shown in the example below.

```csharp
const string inputWithOffset = "2023-08-31T07:00:00+02:00";
const string utcInput = "2023-08-31T07:00:00Z";
const string inputWithoutOffset = "2023-08-31T07:00:00";

DateTimeOffset.ParseExact(inputWithOffset, "yyyy'-'MM'-'dd'T'HH':'mm':'ssK", null, DateTimeStyles.None); //works
DateTimeOffset.ParseExact(utcInput, "yyyy'-'MM'-'dd'T'HH':'mm':'ssK", null, DateTimeStyles.None); //works
DateTimeOffset.ParseExact(inputWithoutOffset, "yyyy'-'MM'-'dd'T'HH':'mm':'ssK", null, DateTimeStyles.None); //works

DateTimeOffset.ParseExact(inputWithOffset, "yyyy'-'MM'-'dd'T'HH':'mm':'sszzz", null, DateTimeStyles.None); //works
DateTimeOffset.ParseExact(utcInput, "yyyy'-'MM'-'dd'T'HH':'mm':'sszzz", null, DateTimeStyles.None); //FormatException
DateTimeOffset.ParseExact(inputWithoutOffset, "yyyy'-'MM'-'dd'T'HH':'mm':'sszzz", null, DateTimeStyles.None); //FormatException
```

→ the "zzz" specifier does not allow the "Z" offset and a missing offset, while the "K" specifier accepts those.

I think this is worth to be mentioned in the documentation.

---
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* ID: dda93f14-f1b5-fde6-e441-302cdac261ab
* Version Independent ID: 96abc7c4-3719-82da-e271-881d62f0640f
* Platform ID: 58fb1aa0-4c4c-dd6c-590b-194d21204c54
* Content: [Custom date and time format strings - .NET](https://learn.microsoft.com/en-us/dotnet/standard/base-types/custom-date-and-time-format-strings#KSpecifier)
* Content Source: [docs/standard/base-types/custom-date-and-time-format-strings.md](https://github.com/dotnet/docs/blob/main/docs/standard/base-types/custom-date-and-time-format-strings.md)
* Service: **dotnet-fundamentals**
* GitHub Login: @adegeo
* Microsoft Alias: **adegeo**