Update documentation for multi-schema install #996

Is your feature request related to a problem? Please describe.
With introduction o context for tests in utPLSQL v3.1.8, it is no longer possible to install multiple instances of utPLSQL on a single database.

There is an undocumented compilation flag SELF_TESTING_INSTALL:TRUE that allows to overcome this issue.

Describe the solution you'd like
Change the name of the flag and make sure to include it in documentation.

Describe alternatives you've considered
Update the install to always install with this feature enabled, so that each schema that contains utPSLQL gets it's own context.
Make sure to reflect this in documentation.