Update TagName docs to include behavior if tag is not found (#109934)

* Clarify documentation for TagList.IndexOf

Adds documentation to TagList.IndexOf that states the behavior when the key was not found.

* Clarify documentation for TagList.IndexOf

Adds documentation to TagList.IndexOf that states the behavior when the key was not found.

Author: mpdelbuono

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/TagList.netcore.cs

@@ -266,6 +266,7 @@ public bool Remove(KeyValuePair<string, object?> item)
         /// Searches for the specified tag and returns the zero-based index of the first occurrence within the entire <see cref="T:System.Diagnostics.TagList" />.
         /// </summary>
         /// <param name="item">The tag to locate in the <see cref="T:System.Diagnostics.TagList" />.</param>
+        /// <returns>The zero-based index of the first occurrence within the <see cref="T:System.Diagnostics.TagList" />, or -1 if there is no such tag.</returns>
         public readonly int IndexOf(KeyValuePair<string, object?> item)
         {
             ReadOnlySpan<KeyValuePair<string, object?>> tags =

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Metrics/TagList.netfx.cs

@@ -416,6 +416,7 @@ public bool Remove(KeyValuePair<string, object?> item)
         /// Searches for the specified tag and returns the zero-based index of the first occurrence within the entire <see cref="T:System.Diagnostics.TagList" />.
         /// </summary>
         /// <param name="item">The tag to locate in the <see cref="T:System.Diagnostics.TagList" />.</param>
+        /// <returns>The zero-based index of the first occurrence within the <see cref="T:System.Diagnostics.TagList" />, or -1 if there is no such tag.</returns>
         public readonly int IndexOf(KeyValuePair<string, object?> item)
         {
             if (_overflowTags is not null)