[ci skip] Refine Action Mailer guide.

* Wrap 80 words if possible.
* Make attachment hash options more readable. (L239-L243)

  Before:

  ![screenshot 2014-06-11 02 59 54](https://cloud.githubusercontent.com/assets/1000669/3235432/a91d0464-f0d2-11e3-9e4a-6af2b2a83f1a.png)

   After:

  ![screenshot 2014-06-11 02 59 36](https://cloud.githubusercontent.com/assets/1000669/3235428/a47b28dc-f0d2-11e3-806c-cfaedd3f15df.png)

* L619 Wrap "location" in code: `"location"`.
* L628 Use actual section name.
* L697 Remove unnecessary `./` when refer to other guide.

remove deprecation call while initializing `ClassCache` object

fix error with long keys in ActiveSupport::Cache::FileStore

[ci skip] Improve enable_starttls_auto documentation (Action Mailer).

[ci skip] Add Docs for strip_insignificant_zeros option in number_to_human

Remove deprecated method ActiveRecord::Base.quoted_locking_column

[ci skip] Correct output and use valid options of number helpers

Fix warnings in tests

```
activerecord/test/cases/adapters/postgresql/range_test.rb:159: warning: (...) interpreted as grouped expression
activerecord/test/cases/finder_test.rb:38: warning: shadowing outer local variable - e
activerecord/test/cases/finder_test.rb:43: warning: shadowing outer local variable - e
```

Keep the types of virtual columns after yaml serialization

remove depricated Validatior#setup

On MySQL and PostgreSQL, the adapter does not type cast virtual columns
for us.

Fix cache_digest rake tasks

Inline PG array type casting helper

Bring cache_digests:* rake tasks up-to-date with the API changes introduced in
637bb726

This is a follow up to #15602 which rendered the guides in a weird state:

  >  You can also specify some options just after the field type between curly
     braces. You can use the following modifiers:

  >  `null`         Allows or disallows `NULL` values in the column.

  >  NOTE: `null` and `default` cannot be specified via command line.

The modifiers are now moved into a separate section. The generator
simply referes to that section.

Related to #15583.

/cc @JuanitoFatas

[ci skip] Rename: Rails Database Migration to Active Record Migration.

This will need to setup permanent redirect for the old guide.

ActiveRecord::FinderMethods.find doesn't pass proc parameter to array

Manual merge of #15584.

Closes #15584.

[ci skip] Add note about type modifiers that cannot be specified in command line.

Use HasAndBelongsToMany instead of HABTM

reuse available collection? check instead of macro

Reflection has an available method that is used to check if the
reflection is a collection. Any :has_many macro is considered a
collection and `collection?` should be used instead of
`macro == :has_many`.

add has_one? method and reuse instead of checking macro

Abstract away use of HABTM macro

By having the `:has_and_belongs_to_many` macro in the `@collection`
we are punishing `:has_many` associations because it has to allocate
the array and check the macro.

@collection is returned to `macro == :has_many` and a new reflection
class `HABTMReflection` is created to handle this case instead.