rts: Clean up RTS flags usage message
Align flag descriptions and acknowledge that some flags may not be
available unless the user linked with -rtsopts (as noted in #20961 (closed)).
Fixes #20961 (closed).
Due to a large amount of spam we do not allow new users to create repositories, they are "external" users. If you are a new user and want to create a repository, for example for forking GHC, open a new issue on ghc/ghc using the "get-verified" issue template
Align flag descriptions and acknowledge that some flags may not be
available unless the user linked with -rtsopts (as noted in #20961 (closed)).
Fixes #20961 (closed).