[DOC] Only document .c in subfolders

`prism` contains `prism/srcs.mk`. RDoc picks it up even though
it is not C/Ruby or anything else recognizable.

It currently doesn't apply to `gc/default` but it could in the future, so I made it
more specific too.

.document

@@ -43,10 +43,10 @@ lib
 ext
 
 # GC is split between gc.rb and gc/default/default.c
-gc/default
+gc/default/*.c
 
 # For `prism`, ruby code is in lib and c in the prism folder
-prism
+prism/*.c
 
 # rdoc files
 NEWS.md