Refactors the migration documentation a bit

doc/migration.md

 
 ## Introduction ##
 
-Migrations on `Tracim` lays on [`gearbox migrate`](http://turbogears.readthedocs.io/en/tg2.3.7/turbogears/migrations.html), which in turn lays on [`alembic`](http://alembic.zzzcomputing.com/en/latest/index.html) which is the migration tool dedicated to `SQLAlchemy`.
+This document is intended to developers.
 
-## Quick summup ##
+Migrations on `Tracim` lays on [`gearbox migrate`](http://turbogears.readthedocs.io/en/tg2.3.7/turbogears/migrations.html), which in turn lays on [`alembic`](http://alembic.zzzcomputing.com/en/latest/index.html) which is the migration tool dedicated to `SQLAlchemy`.
 
-In order to use the `gearbox migrate [...]` commands, change your current directory to be `tracim/` from the root of the project :
+In order to use the `gearbox migrate [...]` commands, change your current directory to be `tracim/` from the root of the project, also usually named `tracim/` :
 
     (tg2env) user@host:~/tracim$ cd tracim/
     (tg2env) user@host:~/tracim/tracim$
 
+## Migration howto - Overview ##
+
+### Upgrading schema ###
+
+    gearbox migrate upgrade
+
+### Downgrading schema ###
+
+    gearbox migrate downgrade
+
+## Migration howto - Advanced (for developers) ##
+
 ### Retrieving schema current version ###
 
     gearbox migrate db_version
 This creates a new python migration file in `tracim/migration/versions/` ending by `migration_label.py`:
 
     gearbox migrate create 'migration label'
-
-### Upgrading schema ###
-
-    gearbox migrate upgrade
-
-### Downgrading schema ###
-
-    gearbox migrate downgrade