Clarify throw vs overwrite semantics for each approach

[KathleenDollard]

The para before the example states that Add vs read/write, but I think a casual user may not pick this up. I would suggest a note here (a callout) that explains that one throws and one overwrites.

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Document Details

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• ID: 38e35585-87f4-f2da-1aa3-5a6b2182f45e
• Version Independent ID: d9b333b1-7e9b-2efe-3284-43ad82826820
• Content: How to initialize a dictionary with a collection initializer - C# Programming Guide
• Content Source: docs/csharp/programming-guide/classes-and-structs/how-to-initialize-a-dictionary-with-a-collection-initializer.md
• Product: dotnet-csharp
• Technology: csharp-fundamentals
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

[BartoszKlonowski]

I'll work on this.