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#27343Fixes#26338
Define the entry point to each mode as an option for the `cmake`
program, but reference the options for that mode as part of stand-in
`cmake-<mode>` programs.
Improve formatting, primarily by:
* Adding links to relevant commands, properties, generators, and so on.
* Converting code, symbols, paths, and so on to fixed-width fonts.
* Hard wrapping lines to 80 characters or less.