My suggestion, therefore, is to add a note to section 3.5.2 of the GHC user guide ("Extra Libraries" in the section on GHCi), along the lines of:

[[ Additional libraries provided as Haskell Source files (.hs, .lhs) may be located by using the "-i" command line option. The directory name must be specified without a trailing "/" (or "\") character. ]]

There's a confusing overlap of terminology here: the GHCi documentation is using the term 'library' to mean a library of object code - files called foo.a or foo.lib, whereas you expect 'library' to mean a source code library. The -i option is documented earlier on, in section 3.2.1 ("The search path for finding source files..."). Cheers, Simon