Doc/ Adding 'exec' keyword info to documentation (#793)

* Added info

* Restructuring.

doc/source/user_guide/launcher.rst

@@ -10,7 +10,7 @@ the :func:`launch_mapdl() <ansys.mapdl.core.launch_mapdl>`:
 
 .. code:: python
 
-    from ansys.mapdl import launch_mapdl
+    from ansys.mapdl.core import launch_mapdl
     mapdl = launch_mapdl()
 
 Python will automatically attempt to detect your MAPDL binary based on
@@ -38,6 +38,35 @@ the path again.  If you need to change the default ansys path
 
 Also see :func:`change_default_ansys_path() <ansys.mapdl.core.change_default_ansys_path>` and :func:`find_ansys() <ansys.mapdl.core.launcher.find_ansys>`.
 
+Additionally, it is possible to specify the executable using the keyword argument ``exec_file``. 
+In Linux:
+
+.. code:: python
+
+    from ansys.mapdl.core import launch_mapdl
+
+    mapdl = launch_mapdl(exec_file='/usr/ansys_inc/v212/ansys/bin/ansys212')
+
+
+And in Windows:
+
+.. code:: python
+
+    from ansys.mapdl.core import launch_mapdl
+
+    mapdl = launch_mapdl(exec_file='C://Program Files//ANSYS Inc//v212//ANSYS//bin//winx64//ansys212.exe')
+
+You could also specify a custom executable by adding the correspondent flag (``-custom``) to the additional switches keyword argument.
+
+.. code:: python
+
+    from ansys.mapdl.core import launch_mapdl
+
+    custom_exec = '/usr/ansys_inc/v212/ansys/bin/ansys212t'
+    add_switch = f" -custom {custom_exec}"
+    mapdl = launch_mapdl(additional_switches=add_switch)
+
+
 
 API Reference
 ~~~~~~~~~~~~~