Document kind of tag that is created from the UI

When creating a tag from the UI without a message, we create a
lightweight tag, versus an annotated tag when a message was provided.

https://git-scm.com/book/en/v2/Git-Basics-Tagging

app/helpers/tags_helper.rb

@@ -28,4 +28,14 @@ module TagsHelper
   def protected_tag?(project, tag)
     ProtectedTag.protected?(project, tag.name)
   end
+
+  def tag_description_help_text
+    text = s_('TagsPage|Optionally, add a message to the tag. Leaving this blank creates '\
+              'a %{link_start}lightweight tag.%{link_end}') % {
+      link_start: '<a href="https://git-scm.com/book/en/v2/Git-Basics-Tagging\" target="_blank" rel="noopener noreferrer">',
+      link_end: '</a>'
+    }
+
+    text.html_safe
+  end
 end

app/views/projects/tags/new.html.haml

@@ -31,7 +31,7 @@
     .col-sm-10
       = text_area_tag :message, @message, required: false, class: 'form-control', rows: 5
       .form-text.text-muted
-        = s_('TagsPage|Optionally, add a message to the tag.')
+        = tag_description_help_text
   %hr
   .form-group.row
     = label_tag :release_description, s_('TagsPage|Release notes'), class: 'col-form-label col-sm-2'

locale/gitlab.pot

@@ -15676,7 +15676,7 @@ msgstr ""
 msgid "TagsPage|New tag"
 msgstr ""
 
-msgid "TagsPage|Optionally, add a message to the tag."
+msgid "TagsPage|Optionally, add a message to the tag. Leaving this blank creates a %{link_start}lightweight tag.%{link_end}"
 msgstr ""
 
 msgid "TagsPage|Optionally, add release notes to the tag. They will be stored in the GitLab database and displayed on the tags page."