docs: document Top(order_by="_name") in hidden-attribute matrix

[dimitri-yatsenko]

Summary

Adds one row to the §3.4 hidden-attribute behavior matrix to document that dj.Top(order_by="_name") flows through to SQL ORDER BY without heading validation.

Why this row is missing today

The current matrix (added in #162) covers reads (fetch/proj/to_dicts/...), writes (insert/update1), restrictions (dict and string), joins, describe(), and indexes — but not Top. Worth a row because Top.order_by behaves like the string-restriction case (passes the column name through to SQL untouched), so a hidden column can legitimately shape the result order even though its value can never return to Python.

Code path

1. Top.__post_init__ (condition.py:142-148) — type-checks order_by is a string or list of strings; no name validation.
2. restrict() (expression.py:241-254) — when the restriction is a Top, short-circuits with return result at line 254, before reaching the heading.names check at lines 260-262 that fires for non-Top conditions.
3. sorting_clauses() (expression.py:133-145) → _flatten_attribute_list (:1580-1604, only special-cases "KEY") → _wrap_attributes (:1607-1617, regex \b((?!asc|desc)\w+)\b quotes the name as `_name`).
4. The quoted hidden-column name lands in ORDER BY. The SELECT list is still built from heading.names at expression.py:163, so the value never returns to Python.

Worked example

# Five most recently populated rows
MyTable & dj.Top(5, order_by="_job_start_time DESC")

Generated SQL (visible columns only in SELECT; hidden column referenced only in ORDER BY):

SELECT <visible cols>
FROM <table>
ORDER BY `_job_start_time` DESC
LIMIT 5

Caveat

Because Top does no name check, a typoed column name passes DataJoint validation and fails as a SQL error from the DB. Not a hidden-attribute concern specifically, but worth knowing the failure mode if you mistype _job_start_tine.

Test plan

• python scripts/gen_llms_full.py regenerates cleanly.
• Build the docs locally and confirm the new row renders correctly in the matrix.