Add mutation fields to GraphQL docs

Add extra null-check for mutation types.

This adds deprecation notices that were missing on mutations and
arguments.

Use English expansion of `e.g.`

Correct another lowercase heading

This includes removing periods after the name of the input type.

app/graphql/mutations/labels/create.rb

@@ -25,7 +25,7 @@ module Mutations
                default_value: Label::DEFAULT_COLOR,
                description: <<~DESC
                  The color of the label given in 6-digit hex notation with leading '#' sign
-                 (e.g. `#FFAABB`) or one of the CSS color names
+                 (for example, `#FFAABB`) or one of the CSS color names
                  <https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#Color_keywords>.
                DESC
 

lib/gitlab/graphql/docs/helper.rb

@@ -63,7 +63,7 @@ module Gitlab
 
         def render_full_field(field, heading_level: 3, owner: nil)
           conn = connection?(field)
-          args = field[:arguments].reject { |f| conn && CONNECTION_ARGS.include?(f[:name]) }
+          args = field[:arguments].reject { |arg| conn && CONNECTION_ARGS.include?(arg[:name]) }
           arg_owner = [owner, field[:name]]
 
           chunks = [
@@ -370,7 +370,7 @@ module Gitlab
           input_field = query[:input_fields]&.first
           return unless input_field
 
-          "Input type: `#{input_field[:type][:name]}`."
+          "Input type: `#{input_field[:type][:name]}`"
         end
 
         def deprecations

lib/gitlab/graphql/docs/templates/default.md.haml

@@ -33,7 +33,7 @@
 :plain
   ## `Mutation` type
 
-  The `Mutation` type contains the API's top-level entry points for all executable mutations.
+  The `Mutation` type contains all the mutations you can execute.
 
   All mutations receive their arguments in a single input object named `input`, and all mutations
   support at least a return field `errors` containing a list of error messages.

spec/lib/gitlab/graphql/docs/renderer_spec.rb

@@ -414,7 +414,7 @@ RSpec.describe Gitlab::Graphql::Docs::Renderer do
 
             Make everything very pretty.
 
-            Input type: `MakeItPrettyInput`.
+            Input type: `MakeItPrettyInput`
 
             #### Arguments