in_http: Improve the documentation for `time` parameter and `<parser>`

[fujimotos]

I belive these two features deserve better publicity, since they have obvious real-world applications. So I applied the following modification:

• Move the description for time parameter to "Basic Usage"
• Add a more descriptive title to the section for <parser>

Also a handful of small improvements are included in this patch. These should allow users to skim through the contents with more ease.