Add `config.active_record.warn_on_records_fetched_greater_than` to the
Configuring Rails Guide.

* Add conjunction for clarity.
* Add comma between independent clauses: create command, and generation
  of fixtures.

- Remove deprecated `[]=`

- Fix duplicate `errors#add` example. The second code example was
originally `[]=`, replace it with `[] <<`.

- Improve explanations for `errors#add` and `errors#full_messages`

Follow-up to PR #19457 (closed after borking my git history). Apologies
for the duplicate PR.

cc @kaspth

Adds documentation for substitutions and why Regexp substitions look different
when assertion fails.

Follow up of https://github.com/rails/rails/pull/19263.

…as discussed #19413

suggested by @senny
https://github.com/rails/rails/pull/18305#issuecomment-73847595

Preserving RACK_ENV behavior.

This reverts commit 7bdc7635, reversing
changes made to 45786be5.

[ci skip]

- Also changed 'a' to 'an' for 'Article' word.

Fixes db:structure:dump when using schema_search_path and PostgreSQL
extensions.

Closes #17157.

getting_started.html#the-mvc-architecture is lost by this commit
2f06c94e (Mar 14 2012 !).

So replace it with wikipedia link.

Closes #19321. [ci skip]

Changed "create Articles view" to "new article view". The create action doesn't typically have a view assigned to it. The view that's being referred to is the 'new' Article view.

  * [ci skip]