add projection syntax clarification

docs/howto_guides/api_gui.md

@@ -95,7 +95,7 @@ The syntax for the filter parameter of the __metadata__ endpoints is slightly di
 | filter | Allows conditions to be set as part of the query, returning only results that satisfy the conditions | [MongoDB-like query language](https://www.mongodb.com/docs/manual/tutorial/query-documents/). All strings should be in double quotation marks. | `{"lat_lon.latitude": {"$gt": 45.0}, "ecosystem_category": "Plants"}` | 
 | max_page_size | Specifies the maximum number of documents returned at a time | Integer | `25`
 | page_token | Specifies the token of the page to return. If unspecified, the first page is returned. To retrieve a subsequent page, the value received as the `next_page_token` from the bottom of the previous results can be provided as a `page_token`. ![next_page_token](../_static/images/howto_guides/api_gui/metadata_page_token_param.png) | String | `nmdc:sys0ae1sh583`
-| projection | Indicates the desired fields to be included in the response. Helpful for trimming down the returned results | Comma separated string of field names that correspond to a `collection_name`. | `name, ecosystem_type` |
+| projection | Indicates the desired fields to be included in the response. Helpful for trimming down the returned results | Comma-separated list of attributes that belong to the documents in the collection being queried | `name, ecosystem_type` |
 | doc_id | The unique identifier of the item being requested. For example, the identifier of a biosample or an extraction | Curie e.g. `prefix:identifier` | `nmdc:bsm-11-ha3vfb58` |<br/>
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