default spin count not specified for constructors that don't explicitly set it

There are three constructors for ManualResetEventSlim. Two don't specify the spin count, while one does. The class is supposed to spin for some amount of time before falling back to a normal wait handle wait. The problem is, I have no clue what the default spin count is, if I don't specify it. By putting in a value, I don't know if I'm speeding it up or slowing it down. The documentation would be better if it could say, "Calling the parameterless constructor makes a new ManualResetEventSlim with a spin count of 10," or if a specific amount isn't something you want to publicly commit to, at least a general idea of what the number is set to, or some hints about the rules that are used to make the decision. Because I don't know what the default is, it's harder to make a decision about which constructor I ought to be using. I suppose my only fallback option is to create it and check the SpinCount property.

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* ID: 094c5713-5b4f-8928-2f63-d6ea692baf27
* Version Independent ID: 0bf6da8d-78a1-9ada-46f7-7e8bc8ae8146
* Content: [ManualResetEventSlim Constructor (System.Threading)](https://docs.microsoft.com/en-us/dotnet/api/system.threading.manualreseteventslim.-ctor?view=netframework-4.7.2#System_Threading_ManualResetEventSlim__ctor)
* Content Source: [xml/System.Threading/ManualResetEventSlim.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Threading/ManualResetEventSlim.xml)
* Product: **.net**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**

Start with xml/System.Threading/ManualResetEventSlim.xml and review the entries for all three constructors, along with the SpinCount property documentation. Confirm what default behavior can be documented for the parameterless constructors, then update the constructor text so users can choose an overload with a clear understanding of its spin-count behavior.

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