Initial work on obtaining the output type description.
<see cref="xxx"/> elements are now rewritten to text.
Applied caching to comment lookups and rewrites.
Fixed typo.
Adds support for explicit documentation of InputTypes.

For any paramater that accepts pipeline input, you can now define a
description in the corresponding <command:inputType> using <para
type="inputType"/> elements in the parameter's XML Doc comments.
Merge branch 'master' into BetterTypeDescriptions
Updated XML Docs.