docs: correct five system-table column descriptions

[jackylee-ch]

Five system-table column descriptions do not match what the code returns:

• $tags.create_time / .time_retained are documented as carrying data, but tags.rs:117
  fills both with new_null_array — the Rust snapshot has no tag creation time or retention.
• $partitions.created_by / updated_by are documented as snapshot ids; they are the audit
  user (spec/partition.rs:40).
• $table_indexes.partition is documented as NULL for unpartitioned tables; it is "{}".
• $physical_files_size counts everything under a bucket directory as data, but
  referenced_files.rs:677 excludes names starting with index-.

The $table_indexes one is the same mistake QuakeWang caught for $files in #669.