ci(doc): improve missing docs workflow

1. Add new pattern `runtime/doc/**`. This is a common case were the
   contributor modifies only the help file but the doc gen would discard
   their changes.

2. Add to the output what the changes after running doc gen would be.

[skip ci]

.github/workflows/api-docs-check.yml

@@ -7,6 +7,7 @@ on:
     paths:
       - 'src/nvim/api/*.[ch]'
       - 'runtime/lua/**.lua'
+      - 'runtime/doc/**'
 
 jobs:
   call-regen-api-docs:

.github/workflows/api-docs.yml

@@ -7,6 +7,7 @@ on:
     paths:
       - 'src/nvim/api/*.[ch]'
       - 'runtime/lua/**.lua'
+      - 'runtime/doc/**'
     branches:
       - 'master'
       - 'release-[0-9]+.[0-9]+'
@@ -56,6 +57,8 @@ jobs:
         if: ${{ steps.docs.outputs.UPDATED_DOCS != 0 && inputs.check_only }}
         run: |
           echo "Job failed, run ./scripts/gen_vimdoc.py and commit your doc changes"
+          echo "The doc generation produces the following changes:"
+          git --no-pager diff
           exit 1
 
       - name: Automatic PR