Update base for Update on "Verify pte size at runtime"

Check that the file size is at least the size of what we expect from the extended header.

ie. header_offset + program_size + segment_data_size.

Differential Revision: [D81938296](https://our.internmc.facebook.com/intern/diff/D81938296/)

[ghstack-poisoned]

Author: lucylq

docs/source/pte-file-format.md

@@ -72,7 +72,8 @@ Optional extended header:
 | [24..31] uint64_t offset (from byte offset zero above) to the start of the
 |          first segment, or zero if there are no segments.
 | [32..39] uint64_t size of the segment data, ie. the size from the segment_base_offset
-|          to the end of the segments.
+|          to the end of the segments. Note, the last segment should not have any
+|          trailing padding.
 | [40..?]  Any zero-padding necessary to preserve the alignment of the data
 |          that follows.
 End of optional extended header.