soong_ui kati.go: write more comments.

This CL also removes --warn_real_to_phony and --warn_phony_looks_real,
since --werror_real_to_phony and --wrror_phony_looks_real are already
specified.

Test: treehugger
Bug: 173474588

Change-Id: Icbb0f53182d1b912c62f8044f52bb79d985bf868