When a built-in cache variable is set via `-D <var>=<value>` with no
help text supplied, populate `HELPSTRING` from the built-in
documentation table (sourced from `Help/variable/*.rst` and the
per-language pattern manuals). Project-defined and unknown variables
continue to receive the legacy "No help, variable specified on the
command line." sentinel.
The cache entry's type and value are left untouched: type stays
`UNINITIALIZED` for bare `-D` and an explicit `-D <var>:TYPE=<value>`
is honored as before. The `cmCacheManager::LoadCache` path is not
modified, so reload of existing caches is a fixed point.
Fixes: #27830
