Clarify that aggregation attributes can be primary or secondary in B

In A.aggr(B), B must have all of A's primary key attributes, but these
can be either primary or secondary attributes in B.

Co-authored-by: dimitri-yatsenko <dimitri@datajoint.com>

Author: claude

docs/src/design/semantic-matching-spec.md

@@ -442,9 +442,9 @@ In `A.aggr(B, ...)`, entries from B are grouped by A's primary key and aggregate
 
 **Functional Dependency Requirement**: Every entry in B must match exactly one entry in A. This requires:
 
-1. **B must have all of A's primary key attributes**: If A's primary key is `(a, b)`, then B must contain attributes named `a` and `b`.
+1. **B must have all of A's primary key attributes**: If A's primary key is `(a, b)`, then B must contain attributes named `a` and `b`. These attributes can be either primary or secondary in B.
 
-2. **Primary key attributes must be homologous**: The namesake attributes in B must have the same lineage as in A. This ensures they represent the same entity.
+2. **These attributes must be homologous**: The namesake attributes in B must have the same lineage as in A. This ensures they represent the same entity.
 
 ```python
 # Valid: Session.aggr(Trial, ...) where Trial has session_id from Session

src/datajoint/expression.py

@@ -469,7 +469,8 @@ def aggr(self, group, *attributes, keep_all_rows=False, **named_attributes):
         In A.aggr(B, ...), groups entries from B by the primary key of A and computes
         aggregate functions. Requires functional dependency: every entry in B must match
         exactly one entry in A. This means B must have all of A's primary key attributes
-        as homologous namesakes (same name AND same lineage).
+        as homologous namesakes (same name AND same lineage). These attributes can be
+        either primary or secondary in B.
 
         :param group: the query expression to aggregate (B in A.aggr(B))
         :param attributes: attributes from self to include in the result
@@ -643,7 +644,8 @@ def create(cls, arg, group, keep_all_rows=False):
 
         For A.aggr(B, ...), ensures functional dependency: every entry in B must match
         exactly one entry in A. This requires B to have all of A's primary key attributes
-        as homologous namesakes (same name AND same lineage).
+        as homologous namesakes (same name AND same lineage). These attributes can be
+        either primary or secondary in B.
 
         :param arg: the grouping expression (A in A.aggr(B))
         :param group: the expression to aggregate (B in A.aggr(B))