[docstrings] fix project-wide English language mistake
Even though 'u' is a wovel, it may be pronounced 'yu' which is a consonant
sound. In such cases precede it with 'a' instead of 'an'.
.. _cwprops:
The property mecanism
---------------------
.. XXX CWProperty and co
Property API
~~~~~~~~~~~~
.. XXX feed me
Registering and using your own property
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. XXX feed me