fix: update explanation of part ids

planning_ai/documents/summary_intro.txt

@@ -1,5 +1,5 @@
 This document provides a summary of each representation received on this public consultation. A Representation ID is provided which corresponds with the ID column in the .json JDi files. A representation contains both a text submission and optionally an attachment which can take varying forms. The tool processes text data only, so does not process images such as photographs, although these can be explored in the original representations submitted. For representations with attachments that are not considered by this summary, these are listed at the end of the report.
 
-Every attachment is split into separate pages for processing, with each page given a Part ID. As such, attachments that have been split by pages into multiple Part IDs will share the same Representations ID. This specification of both Representation ID and Part ID allows the user to link the produced summaries back to the original documents. 
+Large submissions that cover multiple pages are split into component parts for the large language model to process effectively.  For such large submissions each individual page is given a "Part ID" so that a reference can be made between the summary and the relevant part of the submission that the summary relates to. Attachments that have been split by pages into multiple Part IDs will share the same Representations ID. This specification of both Representation ID and Part ID allows the user to link the produced summaries back to the original documents.
 
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