Help: Drop misleading add_custom_command "makefile terms" explanation
The "In makefile terms" explanation is left from very early days of
CMake to help developers migrating from hand-written makefiles.
These days it is often misinterpreted, particularly in the context
of multiple `add_custom_command` calls, to mean that we generate a
single Makefile containing rules for all custom commands. This
leads to an incorrect mental model of the expressed build system.
The actual generated build system may spread the rules across multiple
targets that do not see the each other's file-level rules, which makes
target-level dependencies important too.
Brad King