Add docs about MySQL table comment

[piotr-cz]

In this PR #334 there was added new option to specify MySQL table comment, however it's missing in docs.
I'm not really sure in which place I should add it (probably new hading for MySQL specific options under Creating a table), moreover I"m not really sure my english would be sufficient.

When looking at the MySQL Adapter specific options, there are two undocumented ones that probably should go to docs as well:

• engine default InnoDB
• collation default utf8_general_ci

[robmorgan]

hi @piotr-cz, no your English is good enough. Why don't you give it a shot and maybe I can edit it afterwards?

[piotr-cz]

Here it is: #1028

[robmorgan]

thanks