docs: explain what export routes actually do in OpenAPI documentation (#10836)

public/openapi/read/user/userslug/export/posts.yaml

@@ -2,6 +2,7 @@ get:
   tags:
     - users
   summary: Export a user's posts (.csv)
+  description: This route retrieves an existing export of user's posts. To create one go to `/user/{userslug}/consent`
   parameters:
     - name: userslug
       in: path

public/openapi/read/user/userslug/export/profile.yaml

@@ -2,6 +2,7 @@ get:
   tags:
     - users
   summary: Export a user's profile data (.json)
+  description: This route retrieves an existing export of user's profile data. To create one go to `/user/{userslug}/consent`
   parameters:
     - name: userslug
       in: path

public/openapi/read/user/userslug/export/uploads.yaml

@@ -2,6 +2,7 @@ get:
   tags:
     - users
   summary: Export a user's uploads (.zip)
+  description: This route retrieves an existing export of user's profile data. To create one go to `/user/{userslug}/consent`
   parameters:
     - name: userslug
       in: path