Explain that default_scope also influences update_all

This was not explicitly stated before and I needed to try it out to be
certain. A little explicit statement in the API docs might help here.
Wrap code snippets in +, not backticks, in sdoc

I grepped the source code for code snippets wrapped in backticks in the comments
and replaced the backticks with plus signs so they are correctly displayed in
the Rails documentation.

[ci skip]
Fix select_tag generating tag when set to false.

Fixes https://github.com/rails/rails/issues/17701.
Merge pull request #17695 from claudiob/replace-all-backticks-with-pluses

Wrap code snippets in +, not backticks, in sdoc [ci skip]
Update grammar in Getting Started Guide [ci skip]
Merge pull request #17704 from fgo/patch-13

Update grammar in Getting Started Guide [ci skip]
make it possible to access fixtures excluded by a `default_scope`.

Prior to this patch you'd end up with an error like:

```
ActiveRecord::RecordNotFound: Couldn't find <Model> with 'id'=<id> [WHERE (<default_scope condition>)]
```
Merge pull request #17702 from tgxworld/fix_select_tag_include_blank_false

Fix select_tag generating tag when set to false.
Move 'the' outside of SQLite and Bundler link text [ci skip]
Fix includes on association with a scope containing joins along with conditions
on the joined assoiciation
Creates an ApplicationMailer and layout by default, including html and body tags to reduce spam score
Merge pull request #17705 from fgo/patch-14

Move 'the' outside of SQLite and Bundler link text [ci skip]
Move test that depends on ActiveRecord

This commit moves a test from `test/template` to `test/activerecord` since the
test depends on ActiveRecord.

This matches the documentation from [RUNNING_UNIT_TESTS](https://github.com/rails/rails/blob/f28d1ddd507174ac233b773cc4f35c3c05ad32e7/actionview/RUNNING_UNIT_TESTS.rdoc):

> Test cases in the test/activerecord/ directory depend on having activerecord and sqlite3 installed.
> If Active Record is not in actionview/../activerecord directory, or the sqlite3 rubygem is not installed, these tests are skipped.
> Other tests are runnable from a fresh copy of actionview without any configuration.

---

More details about this commit.

All the tests starting with `require 'active_record_unit'` are already
inside `test/activerecord`, except for the one test this commit moves.

If you don't have `active_record` on your machine, the following command
currently fails on master:

```bash
```
ConnectionAdapter#substitute_at is technically public API...

We can't change the signature without a deprecation cycle.
[ci skip] correct example output and filename in pg guide
Merge pull request #17712 from y-yagi/correct_postgres_guide

[ci skip] correct example output and filename in pg guide
Fixing wrong link in 'Ruby on Rails Security Guide' [ci skip]

The URL

http://www.h-online.com/security/Symantec-reports-first-active-attack-on-a-DSL-router--/news/102352

points to an article titled 'The H is closing down'. The good one is:

http://www.h-online.com/security/news/item/Symantec-reports-first-active-attack-on-a-DSL-router-735883.html
Merge pull request #17716 from javiervidal/fixing_documentation

Fixing wrong link in 'Ruby on Rails Security Guide' [ci skip]
Remove outdated comments [ci skip]

They were introduced in 23b6def; the serial stuff has been removed since a5d80f8
Rename the primary key index when renaming a table in pg

Also checked to make sure this does not affect foreign key constraints.
(It doesn't).

Fixes #12856
Closes #14088