First patch review - scoutapm PECL extension docs

Hi folks,

Please could someone check my patch looks OK before I commit to it? I'm
adding docs for the new `scoutapm` PECL extension.

I've validated it with the local build tools already, and checked the
documentation generated looks ok, but I'd appreciate someone just checking
I've not made any breaking mistakes :)

I've generated an svn diff and put it in a gist, please do let me know if
there's anything else needed:
https://gist.github.com/asgrim/d3f7e5c0cd5d396d323e06c065d8890e

Thanks
James