user-guide: Document -L RTS flag
user-guide: Document -L RTS flag

See #5641.

(cherry picked from commit 923d2151db261cac36577bcaffa6be41c3f374f9)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc