resolves #327 improve epub-chapter-level attribute documentation

README.adoc

@@ -139,6 +139,7 @@ Like other converters, Asciidoctor EPUB3 handles this chunking task by automatic
 
 When `doctype` attribute is set to `book`, each top-level section will become a separate ebook "chapter" file.
 This includes preface, bibliography, appendix, etc.
+This behavior can be configured via `epub-chapter-level` document attribute.
 
 Otherwise, whole document is converted to a single ebook chapter.
 
@@ -299,8 +300,9 @@ _Only applies to a chapter document._
 
 |epub-chapter-level
 |Specify the section level at which to split the EPUB into separate "chapter" files.
+This attribute only affects documents with `:doctype: book`.
 The default is to split into chapters at level-1 sections.
-This option only affects the internal composition of the EPUB, not the way chapters and sections are displayed to users.
+This attribute only affects the internal composition of the EPUB, not the way chapters and sections are displayed to users.
 Some readers may be slow if the chapter files are too large, so for large documents with few level-1 headings, one might want to use a chapter level of 2 or 3.
 
 |series-name, series-volume, series-id