add docstrings for all TypeDecorator methods

[Mike Bayer]

Just had the chance to require overriding TypeDecorator.compare_values and thought, "how useful". Indeed. Too bad nobody else can use it since it has no docstring of any kind.

[Former user]

Work in progress here: https://bitbucket.org/bradallen/sqlalchemy_07_docs

Most of the methods now have docstrings, with the exception of _adapt_expression whose intent is still a bit unclear to me.

[Former user]

I'm still planning to complete this work for the remaining methods, but have gotten sidetracked by work for the time being...will return to this next weekend.

[Mike Bayer]

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[Mike Bayer]

Replying to bradallen:

I'm still planning to complete this work for the remaining methods, but have gotten sidetracked by work for the time being...will return to this next weekend.

oh, I missed this, I thought you were on #1730. If you want to add some detail from your patch that would be fine !

[Mike Bayer]

will reopen for further detail, private methods, etc. the public methods are in, which was my main concern that they be up on the site.