Fix YAML parsing issue that emerged in Ruby 1.9.

In 1.8.7:

	irb(main):001:0> require 'yaml'
	=> true
	irb(main):007:0> YAML.load('foo: [?]')
	=> {"foo"=>["?"]}

In 1.9.3:

	irb(main):002:0> YAML.load('foo: [?]')
	Psych::SyntaxError: (<unknown>): did not find expected ',' or ']' while parsing a flow sequence at line 1 column 6
		from /Users/andrew.dupont/.rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/psych.rb:203:in `parse'
		from /Users/andrew.dupont/.rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/psych.rb:203:in `parse_stream'
		from /Users/andrew.dupont/.rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/psych.rb:151:in `parse'
		from /Users/andrew.dupont/.rbenv/versions/1.9.3-p392/lib/ruby/1.9.1/psych.rb:127:in `load'
		from (irb):2
		from /Users/andrew.dupont/.rbenv/versions/1.9.3-p392/bin/irb:12:in `<main>'

In PDoc, `?` can be an argument type and a return type. When it's an argument type, we end up with `types: [?]` in the YAML file, and though that was OK before Psych existed, it's not OK now. This was preventing documentation from being generated from at least Ruby 1.9.3 onward.

The solution is to put quotes around the types so that YAML knows we mean for them to be strings.

This ends up not being a problem for the return type (`return_value` in the YAML) because it ends up being escaped, as are most of the values in the YAML output.