CMakeDependentOption: improve documentation

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
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