Document extended meaning of createFile task

Author: franzpoeschel

include/openPMD/IO/AbstractIOHandlerImpl.hpp

@@ -187,6 +187,10 @@ class AbstractIOHandlerImpl
   /** Create a new file in physical storage, possibly overriding an existing file.
    *
    * The operation should fail if m_handler->m_frontendAccess is Access::READ_ONLY.
+   * If m_handler->m_frontendAccess is Access::APPEND, a possibly existing file should not
+   * be overwritten.
+   * Instead, written updates should then either occur in-place or in form of new IO steps.
+   * Support for reading is not necessary in Append mode.
    * The new file should be located in m_handler->directory.
    * The new file should have the filename parameters.name.
    * The filename should include the correct corresponding filename extension.