Describe behaviour of getInput (#808)

src/core.ts

@@ -78,7 +78,9 @@ export function addPath(inputPath: string): void {
 }
 
 /**
- * Gets the value of an input.  The value is also trimmed.
+ * Gets the value of an input.
+ * Unless trimWhitespace is set to false in InputOptions, the value is also trimmed.
+ * Returns an empty string if the value is not defined.
  *
  * @param     name     name of the input to get
  * @param     options  optional. See InputOptions.