Clear changes to schema.rb when upgrading

Having local changes is quite common and it is a stumbling block when
upgrading GitLab. Because schema.rb is generated from the actual DB
schema it is OK to clear the local changes.

doc/update/6.x-or-7.x-to-7.3.md

@@ -69,6 +69,7 @@ sudo -u git -H git fetch --all
 For GitLab Community Edition:
 
 ```bash
+sudo -u git -H git checkout -- db/schema.rb # local changes will be restored automatically
 sudo -u git -H git checkout 7-3-stable
 ```
 
@@ -77,6 +78,7 @@ OR
 For GitLab Enterprise Edition:
 
 ```bash
+sudo -u git -H git checkout -- db/schema.rb # local changes will be restored automatically
 sudo -u git -H git checkout 7-3-stable-ee
 ```
 

doc/update/7.2-to-7.3.md

@@ -23,6 +23,7 @@ sudo -u git -H git fetch --all
 For GitLab Community Edition:
 
 ```bash
+sudo -u git -H git checkout -- db/schema.rb # local changes will be restored automatically
 sudo -u git -H git checkout 7-3-stable
 ```
 
@@ -31,6 +32,7 @@ OR
 For GitLab Enterprise Edition:
 
 ```bash
+sudo -u git -H git checkout -- db/schema.rb # local changes will be restored automatically
 sudo -u git -H git checkout 7-3-stable-ee
 ```