State explicitly that these flags are used when driving either compiling
or linking. For linking, document their order w.r.t. link flags
specified by commands.
Fixes: #24662
Previous the wording could be interpreted to mean that the environment
variables like `CXXFLAGS` are used exclusively to initialize the
corresponding cache entries like `CMAKE_CXX_FLAGS`. State clearly
that the value will be used in combination with builtin defaults.
Issue: #23956
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.
Alexander Borsuk