winbuild/README.md: clarify GEN_PDB option

- Document that GEN_PDB option creates an external database.

Ref: https://github.com/curl/curl/issues/7502

winbuild/README.md

@@ -95,7 +95,8 @@ where `<options>` is one or many of:
                                  - Enable loading OpenSSL configuration
                                    automatically, defaults to yes
  - `ENABLE_UNICODE=<yes/no>`     - Enable UNICODE support, defaults to no
- - `GEN_PDB=<yes/no>`            - Generate Program Database (debug symbols for release build)
+ - `GEN_PDB=<yes/no>`            - Generate External Program Database
+                                   (debug symbols for release build)
  - `DEBUG=<yes/no>`              - Debug builds
  - `MACHINE=<x86/x64>`           - Target architecture (default is x86)
  - `CARES_PATH=<path>`           - Custom path for c-ares