winbuild: remove docs from Makefiles and refer to README.md

Reduce risk for conflicting docs and makes it to a single place to fix
and polish.

add these missing options to the readme:

ENABLE_OPENSSL_AUTO_LOAD_CONFIG and ENABLE_UNICODE

clarify ENABLE_SCHANNEL default varies

Fixes #6216
Closes #6227
Co-Authored-by: Jay Satiro