Update doc and add NEWS entry.

* bzrlib/tests/test_plugins.py:
(TestEnvPluginPath.test_disable_core,
TestEnvPluginPath.test_disable_site,
TestEnvPluginPath.test_override_site,
TestEnvPluginPath.test_override_core):

* bzrlib/plugin.py:
(get_standard_plugins_path): Give a better explanantion and don't
break compatibility.

* bzrlib/help_topics/en/configuration.txt: 
(BZR_PLUGIN_PATH): Update doc.

* doc/en/user-guide/plugins.txt: 
Fix doc.

* doc/en/user-guide/writing_a_plugin.txt: 
Fix doc.