Document limitations of Tilt::Cache.

lib/tilt.rb

@@ -78,6 +78,16 @@ def self.current_template
   #     cache.fetch(path, line, options) { Tilt.new(path, line, options) }
   #
   # Subsequent invocations return the already loaded template object.
+  #
+  # @note
+  #   Tilt::Cache is a thin wrapper around Hash.  It has the following
+  #   limitations:
+  #   * Not thread-safe.
+  #   * Size is unbounded.
+  #   * Keys are not copied defensively, and should not be modified after
+  #     being passed to #fetch.  More specifically, the values returned by
+  #     key#hash and key#eql? should not change.
+  #   If this is too limiting for you, use a different cache implementation.
   class Cache
     def initialize
       @cache = {}