This makes it far clearer that <depends> is a list up front instead of
burying the lede because a list is generally "trivially true" in
CMake[1]. Also clarify that <force> is only available as a local
variable and if queried outside of the "scope" of the
cmake_dependent_option call, will get the stored user cache value.
[1] The exception being when the last entry ends in -NOTFOUND.
Suggested-by: Rui Oliveira
Backport: release