clarify behavior of autocommit mode in presence of transaction

[Chris Jerdonek]

As someone relatively new to Django, in reading the ​Database transactions section of the Django docs, I thought a couple things could be clarified or improved.

Namely, it would be helpful to clarify that both (1) the default "autocommit" behavior of immediately committing doesn't occur if a transaction has already been started, and (2) Django's main TestCase class implicitly starts a transaction (thus "suspending" the default autocommit behavior). Not mentioning the latter in the main transaction docs contributed to my running into documentation issue #21540, so I think resolving this issue will help address issue #21540.

I've already written up a proposed patch with suggested wording for the above. I will post it to this issue shortly. Thanks.

[Chris Jerdonek]

I made ​pull request 2191 for this.

[Tim Graham]

(leaving open for Aymeric to take a look)

[Aymeric Augustin]

The suggestions are legitimate but I believe the patch was less precise than the original in several sentences. I'll commit a slightly edited version.

[Aymeric Augustin <aymeric.augustin@…>]

In 798fd59fad241ca8e48ba2a4526935b92736d30d:

Fixed #21836 -- Improved transaction docs about autocommit mode

Clarified that queries in autocommit mode are committed immediately
only if a transaction has not already been started. Added to the
main transaction docs that Django's TestCase class implicitly wraps
its tests in transactions.

[Aymeric Augustin <aymeric.augustin@…>]

In a21fc1c08667dc319601b8195d0a7d3a4b8f2af5:

[1.6.x] Fixed #21836 -- Improved transaction docs about autocommit mode

Clarified that queries in autocommit mode are committed immediately
only if a transaction has not already been started. Added to the
main transaction docs that Django's TestCase class implicitly wraps
its tests in transactions.

Backport of 798fd59f from master.

[Aymeric Augustin <aymeric.augustin@…>]

In a0a6112afc08c7471b70ee5e35170dfee3b488f3:

[1.6.x] Fixed typo. Refs #21836.

Backport of 8e1fc03 from master