Improve documentation for cross-language support (#3674)

* Update doc that describes how to support cross-references between different languages.

Restructured the doc to describe two different approaches, one with `impute` edges
and the second one with `generates` edges.

* Rename Alic => Source, Bob => Target langs.

* Address comments from #3674.

* Fix link.

* Fix asciidoc errors in indexing-generated-code and rename file.

* Fix grammar.

* Update kythe/docs/schema/indexing-generated-code.txt

Co-Authored-By: zrlk <zrlk@users.noreply.github.com>

* Update kythe/docs/schema/indexing-generated-code.txt

Co-Authored-By: zrlk <zrlk@users.noreply.github.com>

* Remove mention of GREEN.
3 files changed