Issue #2941323 by quietone, joachim: StaticMap should document how/whether it...

Issue #2941323 by quietone, joachim: StaticMap should document how/whether it handles source values of NULL, TRUE, FALSE

(cherry picked from commit 45dee9bf)

core/modules/migrate/src/Plugin/migrate/process/StaticMap.php

@@ -105,6 +105,34 @@
  *       'TRUE': to
  * @endcode
  *
+ * A NULL can be mapped. If the value of the source property 'foo' is NULL then
+ * the value of the destination property bar will be 'to'.
+ *
+ * @code
+ * process:
+ *   bar:
+ *     plugin: static_map
+ *     source: foo
+ *     map:
+ *       NULL: to
+ * @endcode
+ *
+ * If your source data contains booleans, the boolean is treated as a numeric 0
+ * or 1. If the value of the source property 'foo' is TRUE then the value of the
+ * destination property bar will be 'bar'. And if the value of the source
+ * property 'foo' is FALSE then the value of the destination property bar will
+ * be 'bar'.
+ *
+ * @code
+ * process:
+ *   bar:
+ *     plugin: static_map
+ *     source: foo
+ *     map:
+ *       0: foo
+ *       1: bar
+ * @endcode
+ *
  * Mapping from a string which contains a period is not supported. A custom
  * process plugin can be written to handle this kind of a transformation.
  * Another option which may be feasible in certain use cases is to first pass