Overhaul the documentation of shared library SONAMEs

Overhaul the documentation of shared library SONAMEs

Provide much more background information about shared library
versioning and how the different symlinks are used.  Clearly
require the SONAME and corresponding binary package name to be
changed if the library ABI changes.  Add more cautions to the
practice of bundling multiple shared libraries together in one
package.  Clarify that the shared library section does not
apply to private libraries and loadable modules.