Fixed #21478 -- Corrected docs for when Field.db_type() is called.
Merge pull request #2094 from coder9042/ticket_21478

Changed documentation for db_type()
Fixed selenium failure.

This fixes (at least according to tests on the ci itself)
admin_widgets.tests.RelatedFieldWidgetSeleniumFirefoxTests.test_foreign_key_using_to_field

(http://ci.djangoproject.com/job/Django/database=mysql_gis,python=python2.7/3792/testReport/junit/admin_widgets.tests/RelatedFieldWidgetSeleniumFirefoxTests/test_foreign_key_using_to_field/)

The cause for this issue seems to be that wait_page_loaded was executed before click fired
and as such no profile got saved (again just an educated guess, but with this fix I can no
longer reproduce it -- fingers crossed).
Added build to flake8.exclude.
Suppressed ResourceWarning in files tests
Fixed #21728 -- Corrected an inadvertent path change in docs/intro/contributing.txt.
Fixed a sentence in the session security docs; thanks claudep.