[1.6.x] Fixed #19395 -- Added a simple example logging config.

Thanks ken.nelson at maclaren.com.

Backport of 69f0249d7b from master
[1.6.x] Fixed #21484 -- Documented that SQL logging does not include "framework level" queries.

Backport of 4d8d76e7a8 from master
[1.6.x] Fixed #21581 -- Fixed a number of issues with collectstatic.

When STATIC_ROOT wasn't set, collectstatic --clear would delete
every files within the current directory and its descendants.

This patch makes the following changes:

Prevent collectstatic from running if STATIC_ROOT isn't set.

Fixed an issue that prevented collectstatic from displaying the
destination directory.

Changed the warning header to notify when the command is run
in dry-run mode.

Backport of 4befb3015c from master
[1.6.x] Django sees more than 1mm hits per hour

Disqus regularly does 40-50k/s

Backport of a502bbb2f0 from master
[1.6.x] Fixed set literals for python 2.6
[1.6.x] Fixed minor typos.

Backport of 44d40ba521 from master
[1.6.x] Fixed typos. Changed grammar.

Backport of 6dd8631617 from master
[1.6.x] Fixed #21722 -- Added a warning for avoiding XSS vulnerabilities when reusing built-in filters.

Thanks Stephen McDonald for the suggestion.

Backport of 07711e9997 from master
[1.6.x] Fixed #21319 -- Added documentation for the Form.fields attribute.

Thanks pydanny for the report. Also, added documentation about
base_fields attribute and its difference with fields attribute.

Backport of ea83102d0f from master
[1.6.x] Fixed #21728 -- Corrected an inadvertent path change in docs/intro/contributing.txt.

Backport of e6800ea136 from master
[1.6.x] Fixed a sentence in the session security docs; thanks claudep.

Backport of 4d27d311f6 from master
[1.6.x] Fixed #21730 -- Removed superfluous db_index in custom auth docs

Thanks rafalp for the report.
Backport of c717ebbfec from master.
Skip distinct_on_tests setUp that fails due to inserting non-unique NULLs
Removed non-nullable, unique issue with many_to_one_regress.tests.ManyToOneRegressionTests.test_related_null_to_field
Skip model_forms.tests.UniqueTest.test_unique_null
Split one_to_one_regress.tests.OneToOneRegressionTests.test_get_reverse_on_unsaved_object to avoid nullable, non-unique issue.
Merge bf757a2f4dad519fac1b4a458376de3a040f5ca8 to fix cache.tests.FileBasedCacheTests.test_zero_timeout
Fixed #21097 - Added DatabaseFeature.can_introspect_autofield
Add 'microsoft' to the list of vendors that do a better job introspecting database fields.