Merge branch 'sb/in-core-index-doc'

Documentation and in-code comments updates.

* sb/in-core-index-doc:
documentation: retire unfinished documentation
cache.h: document add_[file_]to_index
cache.h: document remove_index_entry_at
cache.h: document index_name_pos

diff --cc cache.h
Simple merge

diff --cc read-cache.c
Simple merge