Clarify masked variable message, add link to docs

Update error message "This variable will not be masked" to
"Cannot use Masked Variable with current value"
Add link to masked variables section to error message

app/views/ci/variables/_variable_row.html.haml

@@ -33,7 +33,9 @@
         name: value_input_name,
         placeholder: s_('CiVariables|Input variable value') }
         = value
-      %p.masking-validation-error.gl-field-error.hide= s_("CiVariables|This variable will not be masked")
+      %p.masking-validation-error.gl-field-error.hide
+        = s_("CiVariables|Cannot use Masked Variable with current value")
+        = link_to icon('question-circle'), help_page_path('ci/variables/README', anchor: 'masked-variables'), target: '_blank', rel: 'noopener noreferrer'
     - unless only_key_value
       .ci-variable-body-item.ci-variable-protected-item
         .append-right-default

locale/gitlab.pot

@@ -1785,6 +1785,9 @@ msgstr ""
 msgid "CiStatus|running"
 msgstr ""
 
+msgid "CiVariables|Cannot use Masked Variable with current value"
+msgstr ""
+
 msgid "CiVariables|Input variable key"
 msgstr ""
 
@@ -1794,9 +1797,6 @@ msgstr ""
 msgid "CiVariables|Remove variable row"
 msgstr ""
 
-msgid "CiVariables|This variable will not be masked"
-msgstr ""
-
 msgid "CiVariable|* (All environments)"
 msgstr ""