Document instrumentation event names and emitted payload parameters

[jch]

It'd be nice to have a single place to reference available events. One suggestion is to write it as rdoc in https://github.com/ruby-ldap/ruby-net-ldap/blob/master/lib/net/ldap/instrumentation.rb

Brought up by this mailing list question: https://groups.google.com/forum/#!topic/ruby-ldap/BOnLDfguB6w

[mtodd]

@jch I can handle this, no problem.

I'm :-1: on RDoc formatting and would prefer Markdown moving forward (aiming to replace existing RDocs along the way). Sound reasonable, @jch @schaary?

[jch]

@mtodd fine by me, the two are close enough for me that I don't have a strong preference.