Make -S an alias of --start in changes query REST API

The option had been added with the name `-S`.  Change the name
to `--start` with `-S` as an alias, making it consistent with the
naming in other REST APIs.

Add the option to the documentation.

Remove the documentation of `-P` and `-N`. These options only exist
to support online reindexing from clients with outdated JS, and are
deprecated. Keeping them in the documentation will potentially
cause confusion.

Bug: Issue 2878
Change-Id: I992acf4a3d1aeebb1ef40c6fed1afcb0fe26fa92

gerrit-server/src/main/java/com/google/gerrit/server/query/change/QueryChanges.java

@@ -80,7 +80,7 @@ public class QueryChanges implements RestReadView<TopLevelResource> {
     imp.setSortkeyBefore(key);
   }
 
-  @Option(name = "-S", metaVar = "CNT", usage = "Number of changes to skip")
+  @Option(name = "--start", aliases = {"-S"}, metaVar = "CNT", usage = "Number of changes to skip")
   public void setStart(int start) {
     imp.setStart(start);
   }