As the summary says, one patch per file, detailed below.
contributing.txt: removed unneeded database test settings from the list.
i18n.txt: corrected long-standing error in count of translation variables.
model-api.txt: corrected a few ReST formatting errors. Removed uppercase letters after colons. Changed a mention of __str__ to __unicode__. Removed a double "not". Rephrased an awkward sentence.
overview.txt: removed uppercase letters after colons.
testing.txt: controversial: is it better having an uppercase letter at sentence beginning, or respect an identifier case?
tutorial01.txt: advanced dates from 2005 to 2007.
tutorial02.txt: removed mention of example.com at the top of the admin pages, since it's not there anymore.