Add alt attribute in the doc

Add alt-[field] attribute for <solid-display-img> widget (and a bit of ponctuation).

source/import_documentation/Mixins/widget-mixin.rst

@@ -71,7 +71,7 @@ Attributes
 
 ``default-[field]``
 ~~~~~~~~~~~~~~~~~~~
-    Value displayed for ``[field]`` when it’s empty or not defined
+    Value displayed for ``[field]`` when it’s empty or not defined.
 
 ``widget-[field]``
 ~~~~~~~~~~~~~~~~~~
@@ -95,11 +95,11 @@ Attributes
 
 ``label-[field]``
 ~~~~~~~~~~~~~~~~~
-   Set the label for the ``[field]``
+   Set the label for the ``[field]``.
 
 ``placeholder-[field]``
 ~~~~~~~~~~~~~~~~~~~~~~~
-    Set the placeholder for the ``[field]``
+    Set the placeholder for the ``[field]``.
 
 ``class-[field]``
 ~~~~~~~~~~~~~~~~~
@@ -126,6 +126,11 @@ Attributes
             editable-name
         ></solid-display>
 
+``alt-[field]``
+~~~~~~~~~~~~~~~
+    *Works only with solid-display-img*
+
+    Set the alt for the field ``[field]``.
 
 ``multiple-[field]``
 ~~~~~~~~~~~~~~~~~~~~