Clarify nullability and default value handling

DataJoint handles nullability through the default value syntax:
- Attribute is nullable iff default is NULL
- No separate NOT NULL / NULL modifier needed
- Examples: required, nullable, and default value cases

Co-authored-by: dimitri-yatsenko <[email protected]>

Author: claude

docs/src/design/tables/storage-types-spec.md

@@ -135,8 +135,8 @@ declarative syntax:
 
 | Modifier | Status | DataJoint Alternative |
 |----------|--------|----------------------|
-| `NOT NULL` / `NULL` | ❌ Not allowed | Position above/below `---` determines nullability |
-| `DEFAULT value` | ❌ Not allowed | Use `= value` syntax after type |
+| `NOT NULL` / `NULL` | ❌ Not allowed | Use `= NULL` for nullable; omit default for required |
+| `DEFAULT value` | ❌ Not allowed | Use `= value` syntax before the type |
 | `PRIMARY KEY` | ❌ Not allowed | Position above `---` line |
 | `UNIQUE` | ❌ Not allowed | Use DataJoint index syntax |
 | `COMMENT 'text'` | ❌ Not allowed | Use `# comment` syntax |
@@ -145,6 +145,20 @@ declarative syntax:
 | `AUTO_INCREMENT` | ⚠️ Discouraged | Allowed with native types only, generates warning |
 | `UNSIGNED` | ✅ Allowed | Part of type semantics (use `uint*` core types) |
 
+**Nullability and defaults:** DataJoint handles nullability through the default value syntax.
+An attribute is nullable if and only if its default is `NULL`:
+
+```
+# Required (NOT NULL, no default)
+name : varchar(100)
+
+# Nullable (default is NULL)
+nickname = NULL : varchar(100)
+
+# Required with default value
+status = "active" : varchar(20)
+```
+
 **Auto-increment policy:** DataJoint discourages `AUTO_INCREMENT` / `SERIAL` because:
 - Breaks reproducibility (IDs depend on insertion order)
 - Makes pipelines non-deterministic