Rewrite ActiveRecord::Base#destroy_all docs to remove incorrect info, mention return value, and improve clarity

(The incorrect information is "at least 2*N database queries" -- it's N+1 at best.)

* 'master' of git@github.com:lifo/docrails:
  Complete draft of configuring guide; add configuring guide to index; regenerate HTML.
  FIx bad date in Guide changelog.
  Update table syntax in guides to fix deprecation warnings during build.
  Regenarate html

Merge branch 'form_helpers'

* form_helpers: (30 commits)
  claridy handling missing files
  Bevy of typo corrections
  first stab at the section on file uploads
  Delete todo section
  info on namespaces
  Rearrange  the date and form builder sections
  Flesh out the section on parameter names
  move the fields_for/form builder section higher up
  waffle about parameter names and fields_for
  Clarify fields_for and form builders
  explain fields_for
  First draft of section on parameter names
  Added section on date/time helpers
  country_select url
  typos
  minor clarifications
  Warning about using select on associations
  Fix one last remaining 'we'
  ActiveRecord -> Active Record
  put credits at end, consistent with other guides
  ...

Removed stuff about overriding validate_on_create on the AR Validations and Callbacks Guide, added new section to the Authors page

Fix small typo on the 'The declarative validation helpers' section of AR Validations and Callbacks Guide