Documentation fixes
- Add missing :since: for NondecreasingIndentation and OverlappingInstances
- Remove duplicated descriptions for Safe Haskell flags and UndecidableInstances. Instead, the sections contain a link.
- compare-flags: Also check for options supported by ghci. This uncovered two more that are not documented. The flag -smp was removed.
- Formatting fixes
- Remove the warning about -XNoImplicitPrelude - it was written in 1996, the extension is no longer dangerous.
- Fix misspelled :reverse: flags
Fixes #18958 (closed).
cc @Kleidukos