Help: Document in add_library how to import libraries with SONAME

Brad Kingrequested to merge
brad.king/cmake:doc-imported-soname into master

When using add_library to create an IMPORTED library target, there are a few target properties that are essential to correctly tell CMake about the library file. We already cover IMPORTED_LOCATION and IMPORTED_IMPLIB. Add IMPORTED_SONAME and IMPORTED_NO_SONAME, which are important in certain cases.

Fixes: #22016 (closed)
Backport: release

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