DOC: add doc strings to entity/v1.main.proto

proto/caosdb/entity/v1/main.proto

@@ -540,7 +540,9 @@ message FindQueryResult {
 
 // A single result row of a SELECT query
 message SelectQueryRow {
-  // The cells of the table row contain the value of the SELECT'ed fields.
+  // The cells of the table row contain the value of the SELECT'ed fields. The
+  // order of the cells follows the order of the columns of the
+  // SelectQueryHeader.
   repeated Value cells = 2;
 }
 
@@ -552,6 +554,8 @@ message SelectQueryColumn {
 
 // The header line of the result of a SELECT query
 message SelectQueryHeader {
+  // List of columns in this table. The order defines the order of the cells in
+  // each of the SelectQueryRows.
   repeated SelectQueryColumn columns = 1;
 }