There are several ways of adding help text to an editor GUI.

The 'standard' way of providing help text is to use tooltips:

GUIContent myLabel = new GUIContent("MyShortLabel", "My longer help text");
integerExample = EditorGUILayout.IntField(myLabel, integerExample);

EditorGUIUtility.LookLikeControls() can be used to make small adjustments to label width. (A bit of a hack.)

For longer descriptions you can add a help text box somewhere in the GUI. The following custom style can be used with GUILayout.Label().

// A decent style.  Light grey text inside a border.
helpStyle = new GUIStyle(GUI.skin.box);
helpStyle.wordWrap = true;
helpStyle.alignment = TextAnchor.UpperLeft;

Color c = Color.white;
c.a = 0.75f;
helpStyle.normal.textColor = c;

// Don't use EditorGUILayout.Label()
GUILayout.Label(
    "My extra long multi-line help text."
    , helpStyle
    , GUILayout.ExpandWidth(true));