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>

Help/generator/MSYS Makefiles.rst

@@ -1,7 +1,11 @@
 MSYS Makefiles
 --------------
 
-Generates MSYS makefiles.
+Generates makefiles for use with MSYS ``make`` under the MSYS shell.
 
-The makefiles use /bin/sh as the shell.  They require msys to be
-installed on the machine.
+Use this generator in a MSYS shell prompt and using ``make`` as the build
+tool.  The generated makefiles use ``/bin/sh`` as the shell to launch build
+rules.  They are not compatible with a Windows command prompt.
+
+To build under a Windows command prompt, use the
+:generator:`MinGW Makefiles` generator.

Help/generator/MinGW Makefiles.rst

@@ -1,7 +1,12 @@
 MinGW Makefiles
 ---------------
 
-Generates a make file for use with mingw32-make.
+Generates makefiles for use with ``mingw32-make`` under a Windows command
+prompt.
 
-The makefiles generated use cmd.exe as the shell.  They do not require
-msys or a unix shell.
+Use this generator under a Windows command prompt with MinGW in the ``PATH``
+and using ``mingw32-make`` as the build tool.  The generated makefiles use
+``cmd.exe`` as the shell to launch build rules.  They are not compatible with
+MSYS or a unix shell.
+
+To build under the MSYS shell, use the :generator:`MSYS Makefiles` generator.