Changed grammar for better readability
ARel -> Arel
Added description for mtime
Updated link to peepcode Git screencast
Tried to make the response status code table more readable

By making it the last thing in it's section and adding pseudo row headers
Fixed typo [ci skip]
Clarify arguments of head method
Align table of HTTP status codes with set supported by current Rack
Ruby 1.9 hash syntax for action controller guide
more Ruby 1.9 Hash syntax in our guides
Change 'validates_presence_of' to 'presence: true' 

Of course validates_presence_of is still working but it is not explained in the guide, I think consequently should use 'presence: true' in this documentation.
took out unnecessary wordage, changed "number" to "amount"
without autosave option updated records not save

Emphasizing that without autosave option only new
records are saved and updated records are not saved
updated rdoc to reflect info about readonly attribute
documentation fixes for Array.wrap and AR::Validations::AssociatedValidator
Document the fact you can add_index on new columns
readonly info for save and save!
bump the rails version in bug report template
stdlib -> standard library
fixed typo in select_tag docs