Tweaks to api docs

[aubergine18]

Following are not mentioned on API site:

* log()
* serpent() - a link to the readme would suffice

Also, on the .print() methods, a note that identical messages will be filtered out within x amount of time to help avoid confusion like this.

[ssilk]

I know (and used a lot!) also this:
https://github.com/kikito/inspect.lua/b ... nspect.lua

[Boodals]

serpent and log are now documented in the new Libraries and functions page.
A message explaining the spam prevention in the various print functions would still be nice.