(docs) Update Completions introduction (modelcontextprotocol#1552)

* Update Completions intro to clarify their use for Arguments, and Prompt Name/Resource URI is used as identity.

* prettier

Author: evalstate

docs/specification/draft/server/utilities/completion.mdx

@@ -7,9 +7,9 @@ title: Completion
 <Info>**Protocol Revision**: draft</Info>
 
 The Model Context Protocol (MCP) provides a standardized way for servers to offer
-argument autocompletion suggestions for prompts and resource URIs. This enables rich,
-IDE-like experiences where users receive contextual suggestions while entering argument
-values.
+autocompletion suggestions for the arguments of prompts and resource templates. When
+users are filling in argument values for a specific prompt (identified by name) or
+resource template (identified by URI), servers can provide contextual suggestions.
 
 ## User Interaction Model