Improve documentation of volatile keyword in docs/csharp/language-reference/keywords/volatile.md to clarify what it does and fix some inaccurate info.

-edit by @BillWagner to add document block.

Document Details

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• ID: d226514d-c7cb-0d1b-b9dc-fb791399dc47
• Version Independent ID: aa5d1078-7f89-98ce-7fdc-107c9ed3eebc
• Content: volatile - C# Reference
• Content Source: docs/csharp/language-reference/keywords/volatile.md
• Product: dotnet-csharp
• Technology: csharp-language-reference
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn