Docs: Polish configuration file documentation

Docs: Polish configuration file documentation

* Mostly adding DEFAULT_DOC directive to hide strange default values.
  This wll help us move to different internal values for no-limit etc
  at some point in the future.

* Migrates some access controls to "DEFAULT: none" instead of "deny all".
 This reduces run-time CPU cycles running useless ACL tests.
 NP: some access controls have been left unchanged due to complexity in
     the code testing them (ie icap_access).

* added documentation for several directives which were missing it

* corrected buffered_logs documentation (text by Alex Rousskov)

* Updated cf_gen tool to produce more descriptive error messages.