cmLocalGenerator: Clarify MODULE link flags placeholder population

Clarify comments from commit 8bcf9c7a3e (Add support of "LINKER:" prefix
for artifact creation flags, 2024-10-12, v4.0.0-rc1~522^2~1) about
MODULE library creation flags.  Also avoid falling back to EXECUTABLE
flag placeholders for SHARED or MODULE libraries.  I don't think we
ever populate `CMAKE_<LANG>_CREATE_SHARED_{LIBRARY,MODULE}` from the
value of `CMAKE_<LANG>_LINK_EXECUTALBE`.