update docs for converting the plugin feature to a package

doc/src/Packages_details.rst

@@ -50,6 +50,7 @@ page gives those details.
    * :ref:`MSCG <PKG-MSCG>`
    * :ref:`OPT <PKG-OPT>`
    * :ref:`PERI <PKG-PERI>`
+   * :ref:`PLUGIN <PKG-PLUGIN>`
    * :ref:`POEMS <PKG-POEMS>`
    * :ref:`PYTHON <PKG-PYTHON>`
    * :ref:`QEQ <PKG-QEQ>`
@@ -843,6 +844,28 @@ Foster (UTSA).
 
 ----------
 
+.. _PKG-PLUGIN:
+
+PLUGIN package
+--------------
+
+**Contents:**
+
+A :doc:`plugin <plugin>` command that can load and unload several
+kind of styles in LAMMPS from shared object files at runtime without
+having to recompile and relink LAMMPS.
+
+**Authors:** Axel Kohlmeyer (Temple U)
+
+**Supporting info:**
+
+* src/PLUGIN: filenames -> commands
+* :doc:`plugin command <plugin>`
+* :doc:`Information on writing plugins <Developer_plugins>`
+* examples/plugin
+
+----------
+
 .. _PKG-POEMS:
 
 POEMS package

doc/src/plugin.rst

@@ -60,7 +60,10 @@ The *clear* command will unload all currently loaded plugins.
 Restrictions
 """"""""""""
 
-Plugins are not available on Windows.
+The *plugin* command is part of the PLUGIN package.  It is
+only enabled if LAMMPS was built with that package.
+See the :doc:`Build package <Build_package>` doc page for
+more info. Plugins are not available on Windows.
 
 For the loading of plugins to work the LAMMPS library must be
 :ref:`compiled as a shared library <library>`.  If plugins