Fix document when returning both file and contents in custom importer

[mtsmfm]

ref: pmowrer/node-sass-json-importer#74

I think our document about custom importer is wrong.
It seems node-sass already considers returning both file and contents.

node-sass/src/libsass/src/context.cpp

Lines 512 to 514 in 746759c

	// resolved abs_path should be set by custom importer
	// use the created uniq_path as fallback (maybe enforce)
	std::string path_key(abs_path ? abs_path : uniq_path);