fix some warnings, add some doc

libs/main/system/piplugin.cpp

@@ -45,7 +45,7 @@
  * Application should use class \a PIPluginLoader to load
  * plugin. Main function is \a load(PIString name).
  * "name" is base name of library, %PIPluginLoader
- * try to use sevaral names, <name>, lib<name> and
+ * try to use sevaral names, \<name\>, lib\<name\> and
  * "dll", "so" and "dylib" extensions, depends on system.
  * For example:
  * \code