Clarify that System.Decimal is not IEEE-754 compliant in documentation

[yretenai]

Issue description

Neither the reference docs or the decimal docs point out that the decimal type is not a binary128 or decimal128 value despite being listed among binary32 and binary64 values.

This is evident when we look at the representation of 1.0 in different 128-bit floating point formats (in little endian.)

• binary128: 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xFF, 0x3F
• decimal128: 0x1, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x08, 0x22
• Decimal (GetBits): 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
• Decimal (Memory): 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00

Note that the last 2 bytes of decimal128 are different. Parsing the decimal128 value into System.Decimal either through GetBits or Memory casting will fail the IsValid() check (indicating it's not a Win32 DECIMAL.)

Since decimal128 and binary128 are standardized in the 2008 revision of the IEEE-754 format, this can lead to confusion when referring to decimal128 values (as System.Decimal is not an IEEE decimal float despite being a decimal floating point value.)

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Document Details

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• ID: b24345d9-7aec-14ad-8c3c-56410267849a
• Version Independent ID: f83555d8-9dac-5019-3345-bb0f03e1311d
• Content: Floating-point numeric types - C# reference
• Content Source: docs/csharp/language-reference/builtin-types/floating-point-numeric-types.md
• Product: dotnet-csharp
• Technology: csharp-language-reference
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BillWagner]

ping @tannergooding for a recommendation on the best fix.