DOC: Clarify Python release documentation
DOC: Clarify Python release documentation

Adds language to clarify the difference between ITK wheel archives
which contain loadable Python packages and ITK build archives which
cache build artifacts for use in subsequent module builds.

Kitware/ITK,hjmjohnson/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,hjmjohnson/ITK,Kitware/ITK,Kitware/ITK,Kitware/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,hjmjohnson/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK