Mark the AddLink() operation as stable. (#3887)

Fixes #3865 

Enough SIGs have implemented prototypes without any concern. Also, no
actual need to detect whether links were added before or after Span
creation has received further attention, which previously was a blocker.

specification/trace/api.md

@@ -4,7 +4,7 @@ linkTitle: API
 
 # Tracing API
 
-**Status**: [Stable, Feature-freeze](../document-status.md), except where otherwise specified
+**Status**: [Stable](../document-status.md), except where otherwise specified
 
 <details>
 <summary>Table of Contents</summary>
@@ -543,8 +543,6 @@ Note that [`RecordException`](#record-exception) is a specialized variant of
 
 #### Add Link
 
-**Status**: [Experimental](../document-status.md)
-
 A `Span` MUST have the ability to add `Link`s associated with it after its creation - see [Links](#link).
 `Link`s added after `Span` creation may not be considered by [Samplers](sdk.md#sampler).