Fix copy/paste typo in not_null_constraints.md

This page seems to be mostly copied from the same docs about adding
text limits and as such a couple of sentences were still referring to
text limits.

doc/development/database/not_null_constraints.md

@@ -175,7 +175,7 @@ class CleanupEpicsWithNullDescription < ActiveRecord::Migration[6.0]
 end
 ```
 
-#### Validate the text limit (next release)
+#### Validate the `NOT NULL` constraint (next release)
 
 Validating the `NOT NULL` constraint will scan the whole table and make sure that each record is correct.
 
@@ -201,7 +201,7 @@ end
 
 ## `NOT NULL` constraints on large tables
 
-If you have to clean up a text column for a [high-traffic table](../migration_style_guide.md#high-traffic-tables)
+If you have to clean up a nullable column for a [high-traffic table](../migration_style_guide.md#high-traffic-tables)
 (for example, the `artifacts` in `ci_builds`), your background migration will go on for a while and
 it will need an additional [background migration cleaning up](../background_migrations.md#cleaning-up)
 in the release after adding the data migration.