Doc patches

Hi there,

Both of these should apply cleanly now. I redid the TagHelper again last night due to changes in trunk yesterday.

Documenation Patch for TagHelper, mainly docs, but also removes double stringify_keys for tag_options. http://dev.rubyonrails.org/ticket/6394

Documenation Patch for AssetTagHelper. Docs only, no source modifications. http://dev.rubyonrails.org/ticket/6452