GNUInstallDirs: Add notice about --prefix option

When setting the installation prefix at the installation phase
(`cmake --install`) using the `--prefix` option, the GNU special cases
aren't applicable. This also adds a small clarification when to use the
`--prefix` option at the installation phase.

Briefly mentioned also the CMake presets installDir field,
the CPACK_PACKAGING_INSTALL_PREFIX variable, and the $<INSTALL_PREFIX>
generator expression so user has a better overview.

Fixes #27343
Fixes #26338