Mark comments that should not be in the docs

Some comments that are meant to separate blocks of code in a file show up
on http://api.rubyonrails.org as though they were part of the documentation.

This commit hides those comments from the documentation.

Stems from the discussion with @zzak at https://github.com/voloko/sdoc/issues/79#issuecomment-64158738

[ci skip]

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Here's how the documentation looks *after* and *before* this commit:

![abstract-clearcache](https://cloud.githubusercontent.com/assets/10076/5170473/f18ae26e-73c4-11e4-95b5-baefb7f33264.png)
![mysql2-quotestring](https://cloud.githubusercontent.com/assets/10076/5170474/f18cc84a-73c4-11e4-9086-9c25380ffb1a.png)
![mysql2-active](https://cloud.githubusercontent.com/assets/10076/5170476/f18d56f2-73c4-11e4-8165-ff68c05e1f69.png)
![mysql2-explain](https://cloud.githubusercontent.com/assets/10076/5170477/f18d6750-73c4-11e4-9ca1-4780c3753bb3.png)
![mysql-active](https://cloud.githubusercontent.com/assets/10076/5170472/f18a90f2-73c4-11e4-8c96-539f60cc8959.png)
![mysql-selectrows](https://cloud.githubusercontent.com/assets/10076/5170475/f18d3708-73c4-11e4-89b6-cc0b0b2a70cb.png)
![sqlite3-explain](https://cloud.githubusercontent.com/assets/10076/5170478/f19d93be-73c4-11e4-8568-0d16e17327df.png)