Add description to AlignmentApproach enum

Author: mchowning

aztec/src/main/kotlin/org/wordpress/aztec/AlignmentApproach.kt

@@ -1,3 +1,9 @@
 package org.wordpress.aztec
 
-enum class AlignmentApproach { SPAN_LEVEL, VIEW_LEVEL }
+/**
+ * With [SPAN_LEVEL] any alignment must be specified at the span level. Importantly, this
+ * means that the View's gravity will always be ignored in determining the text's alignment.
+ *
+ * With [VIEW_LEVEL] alignment, the alignment is determined by the View's gravity.
+ */
+enum class AlignmentApproach { SPAN_LEVEL, VIEW_LEVEL }