Clarify metadata field formatting

README.md

@@ -124,13 +124,16 @@ data/ultrafineweb_l1_en_hq/
 | `meta` | `string` | JSON-encoded source and processing metadata |
 | `dataset_index` | `int64` | Dataset-source identifier |
 
-Common keys in `meta` include:
+Common metadata fields include:
 
 - `url`
-- `language`, `language_score`
-- `warc_record_id`, `warc_date`
+- `language`
+- `language_score`
+- `warc_record_id`
+- `warc_date`
 - `source_file`
-- Counts of replacements for email addresses, IP addresses, phone numbers, identity-card numbers, and credit-card numbers
+
+Some records also include counts of replacements for email addresses, IP addresses, phone numbers, identity-card numbers, and credit-card numbers.
 
 ## 🚀 Quick Start