Address inconsistency with gatsby-remark-images (#22666)

Co-authored-by: Laurie Barth <laurie@LauriesrkLaptop.fios-router.home>

docs/docs/working-with-images-in-markdown.md

@@ -185,14 +185,19 @@ Configure the plugins in your `gatsby-config` file. As with the previous example
 
 ### Using the MDX Plugin
 
-The `gatsby-plugin-mdx` plugin will be used in the example below. Put the `gatsby-remark-images` plugin within the `gatsbyRemarkPlugins` option field of `gatsby-plugin-mdx`.
+The below example uses the `gatsby-plugin-mdx` plugin.
+
+`gatsby-remark-images` needs to be both a sub-plugin of `gatsby-plugin-mdx`, included in the `options` field, and a string entry in the plugins array. `gatsby-plugin-sharp` can be included on its own.
+
+`gatsby-source-filesystem` needs to be pointed at wherever you have your images on disk,
 
 > Note: This example configuration assumes your images and Markdown pages are sourced from the same directory. Check out the section on [configuring for different directories](#configuring-for-images-and-posts-in-different-directories) for additional help.
 
 ```js:title=gatsby-config.js
 module.exports = {
   plugins: [
     `gatsby-plugin-sharp`,
+    `gatsby-remark-images`,
     {
       resolve: `gatsby-plugin-mdx`,
       options: {

packages/gatsby-plugin-mdx/README.md

@@ -52,7 +52,7 @@ MDX seeks to make writing with Markdown and JSX simpler while being more express
 Install with npm:
 
 ```shell
-npm install --save gatsby-plugin-mdx @mdx-js/mdx @mdx-js/react
+npm install gatsby-plugin-mdx @mdx-js/mdx @mdx-js/react
 ```
 
 Install with yarn:
@@ -335,6 +335,7 @@ images can be optimized by Gatsby and you should continue using it.
 // gatsby-config.js
 module.exports = {
   plugins: [
+    `gatsby-remark-images`,
     {
       resolve: `gatsby-plugin-mdx`,
       options: {
@@ -358,6 +359,8 @@ Using a string reference is also supported for `gatsbyRemarkPlugins`.
 gatsbyRemarkPlugins: [`gatsby-remark-images`]
 ```
 
+> Note that in the case of `gatsby-remark-images` the plugin needs to be included as both a sub-plugin of gatsby-plugin-mdx and a string entry in the plugins array.
+
 #### Remark plugins
 
 This is a configuration option that is [mirrored from the core MDX