[OpenSER-Docs] Doc Improvements

[Edson]

I know that just now we are dealing with SGML to XML conversion, but it
seams that some point have to be registered:

1- clearer documentation/explanation of the functions internals.
   Specifically what they do and what they changes (SIP messages, internal
   counters/timers, PV, AVP, etc).
2- make very clear which functions break normal script execution (p.ex. see
   the "noisy_ctimer" parameter in TM module" discussion).

And to close a question:

Should the DOC volume include the Doxigen documentation? Where? In a hole
dedicated and separete chapter, or should each module have in it's
documentation another chapter/section just for dealing with this internal
informations? I, particularly would like to see the generated graphs being
merged just after the function/parameter explanation (just for the
external/exported functions/parameters/variables).

Edson.