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