plugin.def: Improve comment for PLUGIN_INCLUDE_FILE.

plugin.def: Improve comment for PLUGIN_INCLUDE_FILE.

2014-03-18  Basile Starynkevitch  <basile@starynkevitch.net>

        * plugin.def: Improve comment for PLUGIN_INCLUDE_FILE.
        * doc/plugins.texi (Plugin callbacks): Mention
        PLUGIN_INCLUDE_FILE.
        Italicize plugin event names in description.  Explain that
        PLUGIN_PRAGMAS has no sense for lto1. Explain
        PLUGIN_INCLUDE_FILE.
        Remind that no GCC functions should be called after
        PLUGIN_FINISH.
        Explain what pragmas with expansion are.

From-SVN: r208660