Document that stack ghci works best only after stack build.

[mihaimaruseac]

This documents the #2790 and #4148 bugs as we decided in #4177 to only
document this behavior for 1.8.1 release and solve them later.

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!