Add documentation explaining how to read Commit and Tag

Since we stopped supporting these types for reading, but their
name is a natural candidate for someone to try and use in code,
explain where they should be looking instead.

Change-Id: I091a1b0ef71b842016020f938ba3161431aab9c9
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/lib/Commit.java

@@ -54,6 +54,10 @@ import java.util.List;
  * Applications should use this object when they need to manually construct a
  * commit and want precise control over its fields. For a higher level interface
  * see {@link org.eclipse.jgit.api.CommitCommand}.
+ *
+ * To read a commit object, construct a {@link org.eclipse.jgit.revwalk.RevWalk}
+ * and obtain a {@link org.eclipse.jgit.revwalk.RevCommit} instance by calling
+ * {@link org.eclipse.jgit.revwalk.RevWalk#parseCommit(AnyObjectId)}.
  */
 public class Commit {
 	private static final ObjectId[] EMPTY_OBJECTID_LIST = new ObjectId[0];

org.eclipse.jgit/src/org/eclipse/jgit/lib/Tag.java

@@ -52,6 +52,10 @@ import org.eclipse.jgit.revwalk.RevObject;
  *
  * Applications should use this object when they need to manually construct a
  * tag and want precise control over its fields.
+ *
+ * To read a tag object, construct a {@link org.eclipse.jgit.revwalk.RevWalk}
+ * and obtain a {@link org.eclipse.jgit.revwalk.RevTag} instance by calling
+ * {@link org.eclipse.jgit.revwalk.RevWalk#parseTag(AnyObjectId)}.
  */
 public class Tag {
 	private ObjectId tagId;