--- a/doc/interpreter/install.txi	Mon Jan 31 17:27:19 2022 +0100
+++ b/doc/interpreter/install.txi	Tue Feb 01 18:56:11 2022 +0100
@@ -213,7 +213,8 @@
 If you wish to build Octave without GNU readline installed, you must use
 the @option{--disable-readline} option when running the configure script.
 
-The following external software packages are optional but recommended:
+The following external software packages are optional.  Octave can be
+built without them but certain features might be missing:
 
 @table @asis
 @item ARPACK
@@ -315,6 +316,12 @@
 default graphics renderer (@qcode{"qt"}) has been Qt-based, which has
 been OpenGL-based.
 
+@item RapidJSON
+A fast JSON parser/generator for C++ with both SAX/DOM style API
+(@url{https://rapidjson.org/}).  RapidJSON is required to read or write
+from or to JSON files with the functions @code{jsondecode} and
+@code{jsonencode}.
+
 @item SuiteSparse
 Sparse matrix factorization library
 (@url{http://faculty.cse.tamu.edu/davis/suitesparse.html}).