docs(nullabe): fix typos

docs/tutorial/nullable.rst

@@ -6,7 +6,7 @@ Nullable variable
    Now in this section we would like to make it possible that it is not necessary to specify a value
    for a variable.
    
-   With rougail, it is possible for a variable's setttings to have no value (nothing, null, None), 
+   With Rougail, it is possible for a variable's settings to have no value (nothing, null, None), 
    that is, neither a default value nor a user assigned value.
    
 .. prerequisites:: Prerequisites
@@ -31,9 +31,8 @@ Nullable variable
 Variable with the value "null"
 ------------------------------------
 
-
 .. note:: It is important to keep in mind that in Rougail a variable is :term:`mandatory` by default, 
-         meaning that it must either have a :term:`default value` set or a :term:`user data` value assigned in the :term:`user data file`\ .
+         meaning that it must either have a :term:`default value` set or a :term:`user data` value assigned for example in a :term:`user data file`\ .
          This is Rougail's default behavior.
 
          Besides, note that the explicit keyword to express the setting "no value, nothing" in YAML is the `null` explicit keyword.
@@ -42,9 +41,9 @@ Here is how we are going to specify a "no value" (nothing, `null`) as a possible
 
 There are a few things to set in the variable. We need to:
 
-- explicitely disable the default :term:`mandatory` behavior of the `address` variable(just by setting it to `false`),
+- explicitely disable the default :term:`mandatory` behavior of the `address` variable (just by setting it to `false`),
 - redefine the `address` variable inside the `socks_proxy` because this family uses the `proxy` type definition,
-- finally set the `address` default value to `null`.
+- finally removes the `address` default value as determined in the `proxy` type definition.
 
 .. extinclude:: https://forge.cloud.silique.fr/stove/rougail-tutorials/raw/commit/v1.1_090/firefox/20-manual.yml
    :language: yaml
@@ -122,7 +121,7 @@ Variables disabled when condition is null
 
    But actually, why did we want a variable to be non-mandatory?
 
-Well, what we want is we need to be able to :term:`disable <disabled>` either the `port` and the `version` in case of the `address` has no value set:
+Well, what we need is to be able to :term:`disable <disabled>` either the `port` and the `version` in case of the `address` has no value set:
    
 .. extinclude:: https://forge.cloud.silique.fr/stove/rougail-tutorials/raw/commit/v1.1_091/firefox/20-manual.yml
    :language: yaml
@@ -184,13 +183,23 @@ For the `disabled` property to trigger, the condition `when: null` must appear,
 pointing to the `_.address` variable, that is the `socks_proxy.address` variable.
 
 
+.. note:: The `_.` notation means the current path (of the current family).
+
+          As we are in the `socks_proxy` family, we can say::
+
+              _.address == socks_proxy.address
+
 .. keypoints:: Key points
 
-   We learned how to define a variable as having no value; it's possible with Rougail.
+   As one of the main objectives of Rougail is to determine if a variable has a value, 
-   It's `default: null` and `mandatory: false`. 
+   therefore the value of a variable is in principle *required*.
+   We say that a value is :term:`mandatory` by default. 
+   However, it is possible to change this default behavior with the `mandatory: false` parameter 
+   explicitely declared in a variable. 
+   In this section we learned how to define a variable as having no value; it's possible with Rougail.
    
-   We also learned to continue using type redefinition, which we learned in the previous section.
+   In this section we also continued using some type redefinition, which we learned in the previous section.
-   And have continued to :term:`disable <disabled>` a variable depending this on 
+   And we have :term:`disabled` a variable depending this on 
    the presence or absence of a value.