Add a hint on the purpose of JGitInternalException to the constructor

In code review we often see uses of JGitInternalException where a high
level GitAPIException would be more appropriate. Hopefully the word
low-level in the constructor comment will lead to fewer such cases.

Change-Id: Id5ec7897535f6c5c5f0bd153fe0ff15c65083474

org.eclipse.jgit/src/org/eclipse/jgit/api/errors/JGitInternalException.java

@@ -60,6 +60,8 @@ public class JGitInternalException extends RuntimeException {
 	private static final long serialVersionUID = 1L;
 
 	/**
+	 * Construct an exception for low-level internal exceptions
+	 *
 	 * @param message
 	 * @param cause
 	 */
@@ -68,6 +70,8 @@ public class JGitInternalException extends RuntimeException {
 	}
 
 	/**
+	 * Construct an exception for low-level internal exceptions
+	 *
 	 * @param message
 	 */
 	public JGitInternalException(String message) {