Drop the monolithic doc string

We now have full sphinx docs as per pytest-dev#14.
pluggy.py is the entire project and it doesn't make much sense to
describe that module's contents other then saying "this is it folks".

pluggy.py

@@ -1,69 +1,3 @@
-"""
-PluginManager, basic initialization and tracing.
-
-pluggy is the cristallized core of plugin management as used
-by some 150 plugins for pytest.
-
-Pluggy uses semantic versioning. Breaking changes are only foreseen for
-Major releases (incremented X in "X.Y.Z").  If you want to use pluggy in
-your project you should thus use a dependency restriction like
-"pluggy>=0.1.0,<1.0" to avoid surprises.
-
-pluggy is concerned with hook specification, hook implementations and hook
-calling.  For any given hook specification a hook call invokes up to N implementations.
-A hook implementation can influence its position and type of execution:
-if attributed "tryfirst" or "trylast" it will be tried to execute
-first or last.  However, if attributed "hookwrapper" an implementation
-can wrap all calls to non-hookwrapper implementations.  A hookwrapper
-can thus execute some code ahead and after the execution of other hooks.
-
-Hook specification is done by way of a regular python function where
-both the function name and the names of all its arguments are significant.
-Each hook implementation function is verified against the original specification
-function, including the names of all its arguments.  To allow for hook specifications
-to evolve over the livetime of a project, hook implementations can
-accept less arguments.  One can thus add new arguments and semantics to
-a hook specification by adding another argument typically without breaking
-existing hook implementations.
-
-The chosen approach is meant to let a hook designer think carefuly about
-which objects are needed by an extension writer.  By contrast, subclass-based
-extension mechanisms often expose a lot more state and behaviour than needed,
-thus restricting future developments.
-
-Pluggy currently consists of functionality for:
-
-- a way to register new hook specifications.  Without a hook
-  specification no hook calling can be performed.
-
-- a registry of plugins which contain hook implementation functions.  It
-  is possible to register plugins for which a hook specification is not yet
-  known and validate all hooks when the system is in a more referentially
-  consistent state.  Setting an "optionalhook" attribution to a hook
-  implementation will avoid PluginValidationError's if a specification
-  is missing.  This allows to have optional integration between plugins.
-
-- a "hook" relay object from which you can launch 1:N calls to
-  registered hook implementation functions
-
-- a mechanism for ordering hook implementation functions
-
-- mechanisms for two different type of 1:N calls: "firstresult" for when
-  the call should stop when the first implementation returns a non-None result.
-  And the other (default) way of guaranteeing that all hook implementations
-  will be called and their non-None result collected.
-
-- mechanisms for "historic" extension points such that all newly
-  registered functions will receive all hook calls that happened
-  before their registration.
-
-- a mechanism for discovering plugin objects which are based on
-  setuptools based entry points.
-
-- a simple tracing mechanism, including tracing of plugin calls and
-  their arguments.
-
-"""
 import sys
 import inspect
 import copy