Add help when docs download times out

[fhunleth]

Before this commit, if there was a timeout when download docs, an error
like this was printed:

$ mix hex.docs fetch elixir
Couldn't find docs for package with name elixir or version 1.18.0

With this commit, the error looks like this and the suggestion helped
me, at least:

$ mix hex.docs fetch elixir
Timed out trying to download docs. Try changing your timeout settings:

    HEX_HTTP_TIMEOUT=120 mix hex.docs fetch elixir 1.18.0

$ HEX_HTTP_TIMEOUT=120 mix hex.docs fetch elixir 1.18.0
Docs fetched: /Users/fhunleth/.hex/docs/hexpm/elixir/1.18.0

[wojtekmach]

Thank you!