docs/library/index: Add important summary of the intro section as warning.

To make them harder to miss.

docs/library/index.rst

 MicroPython libraries
 =====================
 
+.. warning::
+
+   Important summary of this section
+
+   * MicroPython implements a subset of Python functionality for each module.
+   * To ease extensibility, MicroPython versions of standard Python modules
+     usually have ``u`` (micro) prefix.
+   * Any particular MicroPython variant or port may miss any feature/function
+     described in this general documentation, due to resource constraints.
+
+
 This chapter describes modules (function and class libraries) which are built
 into MicroPython. There are a few categories of modules: