doc: Clarify arrow_schema_to_schema requires fields with field id (#1151)

## Which issue does this PR close?


- Closes #1061 .

## What changes are included in this PR?
Add doc for arrow_schema_to_schema



---------

Co-authored-by: Renjie Liu <[email protected]>

Author: jonathanc-n

crates/iceberg/src/arrow/schema.rs

@@ -213,6 +213,10 @@ fn visit_schema<V: ArrowSchemaVisitor>(schema: &ArrowSchema, visitor: &mut V) ->
 }
 
 /// Convert Arrow schema to Iceberg schema.
+///
+/// Iceberg schema fields require a unique field id, and this function assumes that each field
+/// in the provided Arrow schema contains a field id in its metadata. If the metadata is missing
+/// or the field id is not set, the conversion will fail
 pub fn arrow_schema_to_schema(schema: &ArrowSchema) -> Result<Schema> {
     let mut visitor = ArrowSchemaConverter::new();
     visit_schema(schema, &mut visitor)