Of course the "associated object" (base model) cannot be nil, or you wouldn't
be calling this method on it. What concerns the user is that the base model
must be present in the DB.

* collection.create: Clarified the wording (this had always confused me). I used the phrase
"base model" to mean the model that specifies the has_many association (this wording is also
used previously, e.g. in the "Cardinality and associations" section).

* collection.build: You *can* actually call this to add new association records when the base
model is a new record. In ActiveRecord::Associations::AssociationCollection, #create_record
calls #ensure_owner_is_not_new, but #build_record does not.

Added links to web server options to their respective websites to save people the trouble of having to google them.

Updating documentation for Console. Now mentioniing sandbox and including a link to the irb section in the online Pickaxe.

Conflicts:
	activesupport/lib/active_support/core_ext/hash/conversions.rb

Temporarily add submodules to the Rails repo for Arel and Rack::Mount. These submodules will be removed once we add proper Gemfile following to bundler

[#3259 state:resolved]
Signed-off-by: NEloy Duran <eloy.de.enige@gmail.com>