Javadoc: Fix spelling and use wording closer to the type names

Add documentation

Author: garydgregory

src/main/java/org/apache/commons/cli/help/FilterHelpAppendable.java

@@ -20,6 +20,14 @@ Licensed to the Apache Software Foundation (ASF) under one or more
 
 /**
  * An abstract implementation of {@link HelpAppendable} that writes output to an {@link Appendable} instance.
+ * <p>
+ * This class is the superclass of all classes that filter help appendables. These appendable sit on top of an existing appendable (the <em>underlying</em>
+ * appendable) which it uses as its basic sink of data, but possibly transforming the data along the way or providing additional functionality.
+ * </p>
+ * <p>
+ * The class {@code FilterHelpAppendable} itself simply overrides all methods of {@code HelpAppendable} with versions that pass all requests to the underlying
+ * appendable. Subclasses of {@code FilterHelpAppendable} may further override some of these methods as well as provide additional methods and fields.
+ * </p>
  *
  * @since 1.10.0
  */

src/main/java/org/apache/commons/cli/help/HelpAppendable.java

@@ -20,12 +20,15 @@ Licensed to the Apache Software Foundation (ASF) under one or more
 import java.util.Collection;
 
 /**
- * The definition of a semantic scribe. The semantic scribe write string to output based on the semantic meaning of the type of string. e.g. a Paragraph versus
+ * Defines a semantic scribe. The semantic scribe appends text to an output based on the semantic meaning of the type of string. For example, a Paragraph versus
  * a Heading.
  * <p>
- * The representation of the semantics is dependant upon the format being output. For example the plain text output for a paragraph may print the text followed
+ * The representation of the semantics is dependent upon the format being output. For example, the plain text output for a paragraph may print the text followed
  * by two line breaks, while an XHTML output would print the text surrounded by &lt;p&gt; and &lt;/p&gt;.
  * </p>
+ * <p>
+ * Note the {@link Appendable} documentation on the topics of Unicode and threading, these comments also apply here.
+ * </p>
  *
  * @since 1.10.0
  */