Correct doc about Equivalence Key

Null values for Equivalence Key does in fact trigger batching of multiple code actions for `WellKnownFixAllProviders.BatchFixer`. The doc suggested that code actions with `null` values for `EquivalenceKey` are disquallified from batching. But I have verified multiple times that this is not the case.

docs/analyzers/FixAllProvider.md

@@ -18,7 +18,7 @@ Definitions
  - **Code fixer:** An instance of a type derived from `CodeFixProvider` that provides code fixes for compiler and/or analyzer diagnostics.
  - **Code refactoring:** An instance of a type derived from `CodeRefactoringProvider` that provides source code refactorings.
  - **Code action:** An action registered by `CodeFixProvider.RegisterCodeFixesAsync` that performs a code fix OR an action registered by `CodeRefactoringProvider.ComputeRefactoringsAsync` that performs a code refactoring.
- - **Equivalence Key:** A string value representing an equivalence class of all code actions registered by a code fixer or refactoring. Two code actions are treated as equivalent if they have equal non-null `EquivalenceKey` values and were generated by the same code fixer or refactoring.
+ - **Equivalence Key:** A string value representing an equivalence class of all code actions registered by a code fixer or refactoring. Two code actions are treated as equivalent if they have equal `EquivalenceKey` values and were generated by the same code fixer or refactoring.
  - **FixAll provider:** An instance of a type derived from `FixAllProvider` that provides a FixAll occurrences code fix. A FixAll provider is associated with a corresponding code fixer by `CodeFixProvider.GetFixAllProvider` method.
  - **FixAll occurrences code fix:** A code action returned by `FixAllProvider.GetFixAsync`, that fixes all or multiple occurrences of diagnostics fixed by the corresponding code fixer, within a given `FixAllScope`.