Update documentation

Author: Vasileios Karakasis

docs/configure.rst

@@ -76,9 +76,9 @@ Each system is associated with a set of properties, which in this case are the f
   This should be an alphanumeric string (dashes ``-`` are allowed) and it will be used to refer to this system in other contexts.
 * ``descr``: A detailed description of the system.
 * ``hostnames``: This is a list of hostname patterns following the `Python Regular Expression Syntax <https://docs.python.org/3/library/re.html#regular-expression-syntax>`__, which will be used by ReFrame when it tries to automatically select a configuration entry for the current system.
-* ``modules_system``: In our example, this is only defined for Piz Daint and refers to the environment modules system that should be used for loading environment modules on this system.
-  In this case, the classic Tcl implementation of the `environment modules <https://sourceforge.net/projects/modules/files/Modules/modules-3.2.10/>`__.
-  For a complete list of the supported modules systems, see `here <config_reference.html#.systems[].modules_system>`__.
+* ``modules_system``: This refers to the modules management backend which should be used for loading environment modules on this system.
+  Multiple backends are supported, as well as the special ``nomod`` backend which implements the different modules system operations as no-ops.
+  For the complete list of the supported modules systems, see `here <config_reference.html#.systems[].modules_system>`__.
 * ``partitions``: The list of partitions that are defined for this system.
   Each partition is defined as a separate object.
   We devote the rest of this section in system partitions, since they are an essential part of ReFrame's configuration.

docs/tutorial_advanced.rst

@@ -499,7 +499,7 @@ You can then use those resources transparently from within your test.
 To achieve this in our case, we first need to define a ``memory`` resource in the configuration:
 
 .. literalinclude:: ../tutorials/config/settings.py
-   :lines: 31-52,59-75
+   :lines: 31-52,63-79
    :emphasize-lines: 17-22,32-38
 
 Notice that we do not define the resource for all the partitions, but only for those that it makes sense.

docs/tutorial_basics.rst

@@ -336,8 +336,8 @@ Notice, how you can define the actual commands for invoking the C, C++ and Fortr
 As soon as a programming environment defines the different compilers, ReFrame will automatically pick the right compiler based on the source file extension.
 In addition to C, C++ and Fortran programs, ReFrame will recognize the ``.cu`` extension as well and will try to invoke the ``nvcc`` compiler for CUDA programs.
 
-Finally, the new system that we defined may be identified by the hostname ``tresa`` (see the :js:attr:`hostnames` configuration parameter).
-This will help ReFrame to automatically pick the right configuration when running on it.
+Finally, the new system that we defined may be identified by the hostname ``tresa`` (see the :js:attr:`hostnames` configuration parameter) and it will not use any environment modules system (see the :js:attr:`modules_system` configuration parameter).
+The :js:attr:`hostnames` attribute will help ReFrame to automatically pick the right configuration when running on it.
 Notice, how the ``generic`` system matches any hostname, so that it acts as a fallback system.
 
 .. note::