Take advantage of the recursive nature of .gitignore for Makefile entries.

Prefix other entries with '/' to make them non-recursive.
diff --git a/documentation/.gitignore b/documentation/.gitignore
index 6ec0c70..35b5e99 100644
--- a/documentation/.gitignore
+++ b/documentation/.gitignore
@@ -1,2 +1 @@
-Makefile
-html
+/html