Default iteration count should be mentioned for the constructor that is missing it.

Currently programmers need to read the example code to find out that the default iteration count is 1000 (i.e. way too low for 2019). Defaults like these that should change in time - like the PBKDF2 work factor - aren't a good idea anyway, but hiding them in example code is worse.

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#### Document Details

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* ID: 6999edf5-a871-2297-c3a1-bceee0f51d82
* Version Independent ID: 9e628d64-3f80-8ad6-1295-bfe6fc109e8e
* Content: [Rfc2898DeriveBytes Constructor (System.Security.Cryptography)](https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.rfc2898derivebytes.-ctor?view=netframework-4.7.2#System_Security_Cryptography_Rfc2898DeriveBytes__ctor_System_String_System_Byte___)
* Content Source: [xml/System.Security.Cryptography/Rfc2898DeriveBytes.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Security.Cryptography/Rfc2898DeriveBytes.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**