Multiple Components option is not documented in the online manual

[Kleidukos]

Enabling multi-repl: True in cabal.project is apparently not enough for HLS to use the multi-component / multi-repl feature (we really need to unify terminology at some point).

However, a search for multipleComponents in the manual does not raise anything. Fortunately thanks to some digging I could find a post by our beloved @fendor that I half-remember from Discourse but ultimately this needs to be findable in the manual.