docs: add XML comment on FeatureClient (#507)

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## This PR
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- Adds XML comment on FeatureClient class.

### Related Issues
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Fixes #497

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Signed-off-by: Kyle Julian <[email protected]>

Author: kylejuliandev

src/OpenFeature/OpenFeatureClient.cs

@@ -10,7 +10,7 @@
 namespace OpenFeature;
 
 /// <summary>
-///
+/// OpenFeature Client implementation for resolving feature flags and tracking user interactions.
 /// </summary>
 public sealed partial class FeatureClient : IFeatureClient
 {