Clarified prebuilt object documentation.

test cases/prebuilt object/1 basic/meson.build

@@ -26,9 +26,9 @@ elif host.name() == 'freebsd'
 elif host.name() == 'windows'
   id = meson.get_compiler('c').get_id()
   if id == 'gcc'
-    object = 'mingw.obj'
+    object = 'mingw-i386.obj'
   elif id == 'msvc'
-    object = 'msvc.obj'
+    object = 'msvc-i386.obj'
   else
     error('Unknown compiler.')
   endif

test cases/prebuilt object/1 basic/readme.txt

@@ -0,0 +1,10 @@
+This test checks that a pre-existing object file can be used in projects.
+In order to do this, we need prebuilt objects in the source dir. To enable
+a new platform, the source file source.c needs to be compiled and then
+the Meson file updated to use it.
+
+The object needs to be built with no optimization and debug symbols enabled.
+As an example, this is what a compile command with Gcc on x86 Linux would
+look like:
+
+gcc -c -g -o linux-i386.o source.c