Docs: Add query syntax to COPY docs (#7388)

* Docs: Add query syntax to `COPY` docs

* prettier

* Apply suggestions from code review

Co-authored-by: Seth Paydar <29551413+spaydar@users.noreply.github.com>

---------

Co-authored-by: Seth Paydar <29551413+spaydar@users.noreply.github.com>

Author: alamb

docs/source/user-guide/sql/ddl.md

@@ -19,6 +19,9 @@
 
 # DDL
 
+DDL stands for "Data Definition Language" and relates to creating and
+modifying catalog objects such as Tables.
+
 ## CREATE DATABASE
 
 Create catalog with specified name.

docs/source/user-guide/sql/dml.md

@@ -19,14 +19,19 @@
 
 # DML
 
+DML stands for "Data Manipulation Language" and relates to inserting
+and modifying data in tables.
+
 ## COPY
 
-Copy a table to file(s). Supported file formats are `parquet`, `csv`, and `json`.
+Copies the contents of a table or query to file(s). Supported file
+formats are `parquet`, `csv`, and `json` and can be inferred based on
+filename if writing to a single file.
 
 The `PER_THREAD_OUTPUT` option treats `file_name` as a directory and writes a file per thread within it.
 
 <pre>
-COPY <i><b>table_name</i></b> TO '<i><b>file_name</i></b>' [ ( <i><b>option</i></b> [, ... ] ) ]
+COPY { <i><b>table_name</i></b> | <i><b>query</i></b> } TO '<i><b>file_name</i></b>' [ ( <i><b>option</i></b> [, ... ] ) ]
 
 where <i><b>option</i></b> can be one of:
     FORMAT <i><b>format_name</i></b>
@@ -35,14 +40,21 @@ where <i><b>option</i></b> can be one of:
     ROW_GROUP_LIMIT_BYTES <i><b>integer</i></b>
 </pre>
 
+Copy the contents of `source_table` to `file_name.json` in JSON format:
+
 ```sql
 > COPY source_table TO 'file_name.json';
 +-------+
 | count |
 +-------+
 | 2     |
 +-------+
+```
+
+Copy the contents of `source_table` to one or more Parquet formatted
+files in the `dir_name` directory:
 
+```sql
 > COPY source_table TO 'dir_name' (FORMAT parquet, PER_THREAD_OUTPUT true);
 +-------+
 | count |
@@ -51,6 +63,19 @@ where <i><b>option</i></b> can be one of:
 +-------+
 ```
 
+Run the query `SELECT * from source ORDER BY time` and write the
+results (maintaining the order) to a parquet file named
+`output.parquet` with a maximum parquet row group size of 10MB:
+
+```sql
+> COPY (SELECT * from source ORDER BY time) TO 'output.parquet' (ROW_GROUP_LIMIT_BYTES 10000000);
++-------+
+| count |
++-------+
+| 2     |
++-------+
+```
+
 ## INSERT
 
 Insert values into a table.