Remove backwards-compatible overwrite from Entity Linker

This also adds a docstring about overwrite, since it wasn't present.

spacy/pipeline/entity_linker.py

@@ -28,9 +28,6 @@ ActivationsT = Dict[str, Union[List[Ragged], List[str]]]
 
 KNOWLEDGE_BASE_IDS = "kb_ids"
 
-# See #9050
-BACKWARD_OVERWRITE = True
-
 default_model_config = """
 [model]
 @architectures = "spacy.EntityLinker.v2"
@@ -61,7 +58,7 @@ DEFAULT_NEL_MODEL = Config().from_str(default_model_config)["model"]
         "entity_vector_length": 64,
         "get_candidates": {"@misc": "spacy.CandidateGenerator.v1"},
         "get_candidates_batch": {"@misc": "spacy.CandidateBatchGenerator.v1"},
-        "overwrite": True,
+        "overwrite": False,
         "scorer": {"@scorers": "spacy.entity_linker_scorer.v1"},
         "use_gold_ents": True,
         "candidates_batch_size": 1,
@@ -186,7 +183,7 @@ class EntityLinker(TrainablePipe):
         get_candidates_batch: Callable[
             [KnowledgeBase, Iterable[Span]], Iterable[Iterable[Candidate]]
         ],
-        overwrite: bool = BACKWARD_OVERWRITE,
+        overwrite: bool = False,
         scorer: Optional[Callable] = entity_linker_score,
         use_gold_ents: bool,
         candidates_batch_size: int,
@@ -210,6 +207,7 @@ class EntityLinker(TrainablePipe):
             Callable[[KnowledgeBase, Iterable[Span]], Iterable[Iterable[Candidate]]],
             Iterable[Candidate]]
             ): Function that produces a list of candidates, given a certain knowledge base and several textual mentions.
+        overwrite: Whether to overwrite existing non-empty annotations.
         scorer (Optional[Callable]): The scoring method. Defaults to Scorer.score_links.
         use_gold_ents (bool): Whether to copy entities from gold docs or not. If false, another
             component must provide entity annotations.