Help: Factor out "may use generator expressions"

This had already been factored out to `GENEX_NOTE.rst`, but wasn't
applied very widely. Consolidate the language to consistently spell out
(a) the reference to `cmake-buildsystem(7)` and (b) the prose 'with the
syntax "$<...>"'.

To accompany the change, modify cmRST to override redefinitions of
substitutions within a single context instead of appending. Otherwise,
with the new note files, `cmake --help-*` prints repeated fragments
when it collates all scopes of a given field (e.g., all
`prop_*/COMPILE_OPTIONS.rst` from
`cmake --help-property COMPILE_OPTIONS`).