Add test ensuring that all supported flags are documented

Unfortunately, undocumented flags have tended to sneak into the compiler in the past. Let's test this via CI.

Note that there are a still a number of flags that lack documentation. These are listed (and thereby exempted from the check) in docs/users_guide/expected-undocumented-flags.txt. Ideally this list would shrink to become empty in time.

Fixes #17315 (closed).