[devhelp] help: book-format: add a precision

[Sébastien Wilmet <swilmet src gnome org>]

commit f18d9dddf5398c82ac0e7b150b4b5cb1f2491fbe
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Thu Dec 19 14:42:44 2019 +0100

    help: book-format: add a precision

 help/C/book-format.page | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/help/C/book-format.page b/help/C/book-format.page
index 002dd86a..0b4d8dd8 100644
--- a/help/C/book-format.page
+++ b/help/C/book-format.page
@@ -18,8 +18,9 @@
 </synopsis>
 
 <p>
-  The content of a book is placed in one directory. That directory is
-  comprised of:
+  The content of a book is placed in one directory (that directory contains
+  only one book, it cannot contain several books). The directory is comprised
+  of:
 </p>
 <list>
   <item><p>