Begin the scope of a prefix-URI Namespace mapping.
The information from this event is not necessary for normal
Namespace processing: the XmlParser will automatically replace
prefixes for element and attribute names.
There are cases, however, when applications need to use
prefixes in character data or in attribute values, where they
cannot safely be expanded automatically; the
start/endPrefixMapping event supplies the information to the
application to expand prefixes in those contexts itself, if
necessary.
Note that start/endPrefixMapping events are not guaranteed to
be properly nested relative to each-other: all
startPrefixMapping events will occur before the corresponding
startElementNS event, and all endPrefixMapping events will occur
after the corresponding endElementNS event, but their order is
not guaranteed.