docs: document upgrade and internal type/predicate namespace (#6149) (#6164)

Fixes DGRAPH-1714.
This PR adds documentation for upgrading from `v20.03.x` to `v20.07.0`. It also adds documentation for the reserved `dgraph.` namespace for internal types and predicates.

(cherry picked from commit 0b4cd73)

Author: abhimanyusinghgaur

wiki/content/deploy/dgraph-administration.md

@@ -200,6 +200,30 @@ When the new cluster (that uses the upgraded version of Dgraph) is up and runnin
 dgraph upgrade --acl -a localhost:9080 -u groot -p password
 ```
 
+### Upgrading from v20.03.0 to v20.07.0 for Enterprise Customers
+1. Use [binary]({{< relref "enterprise-features/binary-backups.md">}}) backup to export data from old cluster
+2. Ensure it is successful
+3. [Shutdown Dgraph]({{< relref "#shutting-down-database" >}}) and wait for all writes to complete
+4. Upgrade `dgraph` binary to `v20.07.0`
+5. [Restore]({{< relref "enterprise-features/binary-backups.md#restore-from-backup">}}) from the backups using upgraded `dgraph` binary
+6. Start a new Dgraph cluster using the restored data directories
+7. Upgrade ACL data using the following command:
+    ```
+    dgraph upgrade --acl -a localhost:9080 -u groot -p password -f v20.03.0 -t v20.07.0
+    ```
+    This is required because previously the type-names `User`, `Group` and `Rule` were used by ACL.
+    They have now been renamed as `dgraph.type.User`, `dgraph.type.Group` and `dgraph.type.Rule`, to
+    keep them in dgraph's internal namespace. This upgrade just changes the type-names for the ACL
+    nodes to the new type-names.
+    
+    You can use `--dry-run` option in `dgraph upgrade` command to see a dry run of what the upgrade
+    command will do.
+8. If you have types or predicates in your schema whose names start with `dgraph.`, then
+you would need to manually alter schema to change their names to something else which isn't
+prefixed with `dgraph.`, and also do mutations to change the value of `dgraph.type` edge to the
+new type name and copy data from old predicate name to new predicate name for all the nodes which
+are affected. Then, you can drop the old types and predicates from DB.
+
 {{% notice "note" %}}
 If you are upgrading from v1.0, please make sure you follow the schema migration steps described in [this section](/howto/#schema-types-scalar-uid-and-list-uid).
 {{% /notice %}}

wiki/content/query-language/schema.md

@@ -82,6 +82,10 @@ If data exists and new indices are specified in a schema mutation, any index not
 
 Reverse edges are also computed if specified by a schema mutation.
 
+{{% notice "note" %}}You can't define predicate names starting with `dgraph.`, it is reserved as the
+namespace for Dgraph's internal types/predicates. For example, defining `dgraph.name` as a
+predicate is invalid.{{% /notice  %}}
+
 
 ## Indexes in Background
 

wiki/content/query-language/type-system.md

@@ -23,6 +23,10 @@ type Student {
 }
 ```
 
+{{% notice "note" %}}You can't define type names starting with `dgraph.`, it is reserved as the
+namespace for Dgraph's internal types/predicates. For example, defining `dgraph.Student` as a
+type is invalid.{{% /notice  %}}
+
 Types are declared along with the schema using the Alter endpoint. In order to
 properly support the above type, a predicate for each of the attributes
 in the type is also needed, such as: