org.texi: Update documentation about included files

* doc/org.texi (Include files): Remove reference to :prefix1
  and :prefix.  Give more details for :minlevel.

doc/org.texi

@@ -9703,15 +9703,18 @@ The optional second and third parameter are the markup (e.g., @samp{quote},
 @samp{example}, or @samp{src}), and, if the markup is @samp{src}, the
 language for formatting the contents.  The markup is optional; if it is not
 given, the text will be assumed to be in Org mode format and will be
-processed normally.  The include line will also allow additional keyword
-parameters @code{:prefix1} and @code{:prefix} to specify prefixes for the
-first line and for each following line, @code{:minlevel} in order to get
-Org mode content demoted to a specified level, as well as any options
-accepted by the selected markup.  For example, to include a file as an item,
-use
+processed normally.
+
+Contents of the included file will belong to the same structure (headline,
+item) containing the INCLUDE keyword.  In particular, headlines within the
+file will become children of the current section.  That behaviour can be
+changed by providing an additional keyword parameter, @code{:minlevel}.  In
+that case, all headlines in the included file will be shifted so the one with
+the lowest level reaches that specified level.  For example, to make a file
+become a sibling of the current top-level headline, use
 
 @example
-#+INCLUDE: "~/snippets/xx" :prefix1 "   + " :prefix "     "
+#+INCLUDE: "~/my-book/chapter2.org" :minlevel 1
 @end example
 
 You can also include a portion of a file by specifying a lines range using