Summary

• Update the WD_BUILTIN_STYLE example to look up "Body Text" by style name.
• Clarify that built-in styles must be defined in the document and that WD_STYLE members are not style lookup keys.

Fixes #1439

Validation

• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD
• git show --check --oneline HEAD -- src/docx/enum/style.py docs/api/enum/WdBuiltinStyle.rst
• Not run locally