Tighten some ruby ignore patterns

Ruby.gitignore

@@ -3,21 +3,21 @@
 .config
 coverage
 InstalledFiles
-pkg
-rdoc
-spec/reports
-test/tmp
-test/version_tmp
-tmp
+/pkg/
+/spec/reports
+/test/tmp
+/test/version_tmp
+/tmp/
 
-# YARD artifacts
-.yardoc
-_yardoc
-doc/
+## Documentation cache and generated files:
+/.yardoc
+/_yardoc
+/doc/
+/rdoc
 
 ## Environment normalisation:
-.bundle
-lib/bundler/man
+/.bundle
+/lib/bundler/man
 
 # for a library or gem, you might want to ignore these files since the code is
 # intended to run in multiple environments; otherwise, check them in: