Follow new type conversion specification

[nobu-k]

We decided to provide strict type conversion rules and some specifications that have already been written in the document might change. For example, int vs float comparison, logical operators' return values, and so on.

[nobu-k]

sensorbee/sensorbee#38
Apply this to docs

[nobu-k]

The new timestamp::int rule should be documented.

[nobu-k]

timestamp::int done!