Help: Fix description of `CMAKE_*_LINKER_FLAGS` variables

Improve the wording on the behavior of the per-configuration variants
of `CMAKE_*_LINKER_FLAGS`. In particular, note that these variables
only apply to the specified build configuration, and replace the
ambiguous reference to `CMAKE_C_FLAGS_*` with a precise description.

Clarify that the "default" variants of these variables apply for all
build configurations.

Add links between pages for the default and per-configuration variables.