Help: Clarify that variable references may use cache entries

Clarify in `cmake-language(7)` that a variable lookup may use a cache
entry if the variable is not set.  Fix misleading text in added by
commit 8ba2a8d4a4 (Help: short/long variable reference in if command,
2018-11-01, v3.14.0-rc1~408^2~2).

Fixes: #23863