doc/requirements.in: clarify they are additional, not standalone

Rephrase the file header to make sure these dependencies cannot be
misinterpreted as standalone.

The doc requirements were standalone in January 2021 as demonstrated by
commit e21ffe6 ("requirements-doc: add PyYAML which removes
dependency on -base") and were most likely tested as standalone by CI
until January 2025 when commit b6922c8 ("ci: doc: add
action-zephyr-setup") added a Zephyr setup step to
`.github/workflows/doc-build.yml`

Signed-off-by: Marc Herbert <[email protected]>

Author: marc-hb

doc/requirements.in

@@ -1,4 +1,5 @@
-# DOC: used to generate docs
+# DOC: additional dependencies required to generate docs.
+#      Follow the "Getting Started" guide first.
 
 sphinx<8.3.0
 sphinx_rtd_theme~=3.0