Missing elements in sphinx documentation (mongodb) (#2260)

doc/sources/admin/mongodbconfbackend.rst

@@ -6,6 +6,18 @@ used both for storing configuration and
 :doc:`sessions<mongodbsessionbackend>`. You need to install Perl MongoDB
 module to be able to use this backend.
 
+For Debian, you can install mongodb module with:
+
+::
+
+   apt install libmongodb-perl
+
+For CentOS:
+
+::
+
+   yum install perl-MongoDB
+
 See :doc:`how to change configuration backend<changeconfbackend>` to
 change your configuration database.
 

doc/sources/admin/mongodbsessionbackend.rst

@@ -20,6 +20,21 @@ Perl module (version ⩾ 0.15 required). You also need a recent version of
 client <http://search.cpan.org/~mongodb/MongoDB-v1.2.2/>`__ (version ⩾
 1.00 required).
 
+For Debian, you can install mongodb module and Apache::Session module with:
+
+::
+
+   apt install libmongodb-perl
+   cpan Apache::Session::MongoDB
+
+For CentOS:
+
+::
+
+   yum install perl-MongoDB
+   cpan Apache::Session::MongoDB
+
+
 In the manager: set
 `Apache::Session::MongoDB <http://search.cpan.org/perldoc?Apache::Session::MongoDB>`__
 in ``General parameters`` » ``Sessions`` » ``Session storage`` »