Add note about similarity function in "similar" command's doc (#774)

* note about similarity function in similar command doc
* Link to Wikipedia definition

---------

Co-authored-by: Simon Willison <swillison@gmail.com>

docs/embeddings/cli.md

@@ -325,7 +325,7 @@ llm embed-multi photos \
 (embeddings-cli-similar)=
 ## llm similar
 
-The `llm similar` command searches a collection of embeddings for the items that are most similar to a given or item ID.
+The `llm similar` command searches a collection of embeddings for the items that are most similar to a given or item ID, based on [cosine similarity](https://en.wikipedia.org/wiki/Cosine_similarity).
 
 This currently uses a slow brute-force approach which does not scale well to large collections. See [issue 216](https://github.com/simonw/llm/issues/216) for plans to add a more scalable approach via vector indexes provided by plugins.
 
@@ -419,4 +419,4 @@ llm collections delete collection-name
 Pass `-d` to specify a different database file:
 ```bash
 llm collections delete collection-name -d my-embeddings.db
-```
+```

docs/help.md

@@ -77,7 +77,7 @@ Commands:
   models        Manage available models
   openai        Commands for working directly with the OpenAI API
   plugins       List installed plugins
-  similar       Return top N similar IDs from a collection
+  similar       Return top N similar IDs from a collection using cosine...
   templates     Manage stored prompt templates
   uninstall     Uninstall Python packages from the LLM environment
 ```
@@ -591,7 +591,7 @@ Options:
 ```
 Usage: llm similar [OPTIONS] COLLECTION [ID]
 
-  Return top N similar IDs from a collection
+  Return top N similar IDs from a collection using cosine similarity.
 
   Example usage:
 

llm/cli.py

@@ -1820,7 +1820,7 @@ def embed_multi(
 )
 def similar(collection, id, input, content, binary, number, database):
     """
-    Return top N similar IDs from a collection
+    Return top N similar IDs from a collection using cosine similarity.
 
     Example usage: