Document Schema_XXX classes

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Iaf6205338406002f72531f043191f3dbcddffd02

java/com/google/gerrit/server/schema/Schema_180.java

diff --git a/java/com/google/gerrit/server/schema/Schema_180.java b/java/com/google/gerrit/server/schema/Schema_180.java
index 6912b3e..7e13d0d 100644
--- a/java/com/google/gerrit/server/schema/Schema_180.java
+++ b/java/com/google/gerrit/server/schema/Schema_180.java
@@ -14,6 +14,15 @@
 
 package com.google.gerrit.server.schema;
 
+/**
+ * Schema 180 for Gerrit metadata.
+ *
+ * <p>180 is the first schema version that is supported by NoteDb. All previous schema versions were
+ * for ReviewDb. Since ReviewDb no longer exists those schema versions have been deleted.
+ *
+ * <p>Upgrading to this schema version creates the {@code refs/meta/version} ref in NoteDb that
+ * stores the number of the current schema version.
+ */
 public class Schema_180 implements NoteDbSchemaVersion {
   @Override
   public void upgrade(Arguments args, UpdateUI ui) {

java/com/google/gerrit/server/schema/Schema_181.java

diff --git a/java/com/google/gerrit/server/schema/Schema_181.java b/java/com/google/gerrit/server/schema/Schema_181.java
index 5a238ef..d357ae7 100644
--- a/java/com/google/gerrit/server/schema/Schema_181.java
+++ b/java/com/google/gerrit/server/schema/Schema_181.java
@@ -17,6 +17,12 @@
 import com.google.gerrit.gpg.PublicKeyStore;
 import org.eclipse.jgit.lib.Repository;
 
+/**
+ * Schema 181 for Gerrit metadata.
+ *
+ * <p>Upgrading to this schema version populates the GPG subkey to master key map (see {@link
+ * PublicKeyStore}.
+ */
 public class Schema_181 implements NoteDbSchemaVersion {
   @Override
   public void upgrade(Arguments args, UpdateUI ui) throws Exception {