Help: Clarify MinGW v. MSYS Makefiles generators (#15319)

Explain the usage of each generator more clearly and reference each as
an alternative to the other.

Suggested-by: Craig Hicks <hicks111@hotmail.com>
Merge topic 'doc-mingw-makefiles' into next

267f613f Help: Clarify MinGW v. MSYS Makefiles generators (#15319)