Users guide refers to non-existent -pgmo flag

It appears to be related to iserv-proxy.

Providing :ghc-flag:`-pgmi /path/to/iserv-proxy <-pgmi>`, :ghc-flag:`-pgmo ⟨option⟩`
and :ghc-flag:`-pgmo ⟨port⟩` in addition to :ghc-flag:`-fexternal-interpreter`
will then make ghc go through the proxy instead.

However, DynFlags knows nothing of this flag.

Ccing @angerman

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information