doc: replace DOS with MSYS shell to be more consistent

A plain DOS window doesn't have the right path settings, whereas
the MSYS shell window does.

Conflicts:
    doc/build-msw.md

Rebased-from: 51e3e0909eb8567cde3fe1be0badbc266cb4cc6a 0.8.x

doc/build-msw.md

@@ -63,7 +63,7 @@ MSYS shell:
 
 Boost
 -----
-DOS prompt:
+MSYS shell:
 
 	downloaded boost jam 3.1.18
 	cd \boost-1.50.0-mgw
@@ -82,7 +82,7 @@ MSYS shell:
 
 Litecoin
 -------
-DOS prompt:
+MSYS shell:
 
 	cd \litecoin\src
 	mingw32-make -f makefile.mingw