Modify the cut/coalesce example in Zed language docs (#4776)

docs/language/operators/cut.md

@@ -74,13 +74,19 @@ echo '1 {a:1,b:2,c:3}' | zq -z 'cut a,b' -
 {a:error("missing"),b:error("missing")}
 {a:1,b:2}
 ```
-_Set default values for fields with null values_
-This can be helpful when using the `csv` export format, which expects a uniform record structure (which includes field types).
+_Invoke a function while cutting to set a default value for a field_
+
+:::tip
+This can be helpful to transform data into a uniform record type, such as if
+the output will be exported in formats such as `csv` or `parquet` (see also:
+[`fuse`](fuse.md)).
+:::
+
 ```mdtest-command
 echo '{a:1,b:null}{a:1,b:2}' | zq -z 'cut a,b:=coalesce(b, 0)' -
 ```
 =>
 ```mdtest-output
 {a:1,b:0}
 {a:1,b:2}
-```
+```