[MERGE] Add documentation for hooks

[Ian Clatworthy]

Aaron Bentley wrote:
> Hi all,
> 
> This patch adds some documentation on hooks.  I needed to find out more
> about hooks, and I couldn't find any documentation, so I wrote some.

bb: comment

I'd rather this was part of the Reference Manual instead of the User
Guide. Or perhaps split the content up to achieve that? Either way,
thanks for this - it's been a glaring hole in our documentation.

Ian C.