gnu: gnucash: Add information about how to use the documentation.

It was not obvious that another application, Yelp, needed to be installed to
so that opening the help of GnuCash from within the application would work.

Fixes issue #33960 (see: https://issues.guix.info/issue/33960).

* gnu/packages/gnucash.scm (gnucash)[description]: Explain how to enable the
  documentation.

gnu/packages/gnucash.scm

@@ -188,7 +188,10 @@ (define-public gnucash
 It can be used to track bank accounts, stocks, income and expenses, based on
 the double-entry accounting practice.  It includes support for QIF/OFX/HBCI
 import and transaction matching.  It also automates several tasks, such as
-financial calculations or scheduled transactions.")
+financial calculations or scheduled transactions.
+
+To make the GnuCash documentation available, its doc output must be
+installed as well as Yelp, the Gnome help browser.")
     (license license:gpl3+)))
 
 ;; This package is not public, since we use it to build the "doc" output of