[docs] Clarify .md handling with @next/mdx (#49785)

…iles as MDX

I got bitten by this passage because it reads as if merely changing the
`extension` setting will cause `.md` files to be loaded as MDX.

See: mdx-js/mdx#2302

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Co-authored-by: JJ Kasper <jj@jjsweb.site>

packages/next-mdx/readme.md

@@ -51,7 +51,8 @@ module.exports = withMDX({
 })
 ```
 
-Optionally you can match other file extensions for MDX compilation, by default only `.mdx` is supported
+By default MDX will only match and compile MDX files with the `.mdx` extension.
+However, it can also be optionally configured to handle markdown files with the `.md` extension, as shown below:
 
 ```js
 // next.config.js
@@ -61,6 +62,8 @@ const withMDX = require('@next/mdx')({
 module.exports = withMDX()
 ```
 
+In addition, MDX can be customized with compiler options, see the [mdx documentation](https://mdxjs.com/packages/mdx/#compilefile-options) for details on supported options.
+
 ## Top level .mdx pages
 
 Define the `pageExtensions` option to have Next.js handle `.md` and `.mdx` files in the `pages` directory as pages: