Merge branch 'bw-tweak-migration-style-guide-docs' into 'master'

make a little more obvious what operations might need downtime or enhanced migration techniques

See merge request gitlab-org/gitlab-ce!17927

doc/development/migration_style_guide.md

@@ -23,10 +23,6 @@ When downtime is necessary the migration has to be approved by:
 An up-to-date list of people holding these titles can be found at
 <https://about.gitlab.com/team/>.
 
-The document ["What Requires Downtime?"](what_requires_downtime.md) specifies
-various database operations, whether they require downtime and how to
-work around that whenever possible.
-
 When writing your migrations, also consider that databases might have stale data
 or inconsistencies and guard for that. Try to make as few assumptions as
 possible about the state of the database.
@@ -41,6 +37,18 @@ Migrations that make changes to the database schema (e.g. adding a column) can
 only be added in the monthly release, patch releases may only contain data
 migrations _unless_ schema changes are absolutely required to solve a problem.
 
+## What Requires Downtime?
+
+The document ["What Requires Downtime?"](what_requires_downtime.md) specifies
+various database operations, such as 
+
+- [adding, dropping, and renaming columns](what_requires_downtime.md#adding-columns)
+- [changing column constraints and types](what_requires_downtime.md#changing-column-constraints)
+- [adding and dropping indexes, tables, and foreign keys](what_requires_downtime.md#adding-indexes)
+
+and whether they require downtime and how to work around that whenever possible.
+
+
 ## Downtime Tagging
 
 Every migration must specify if it requires downtime or not, and if it should