plugin: documentation for PluginMetadata updated

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: documentation for PluginMetadataReader updated

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: documentation for PluginMetadataReaderException added

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: documentation for PluginMetadataFinder added

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
plugin: documentation for PluginMetadataBuilder added

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>