documentation: retire unfinished documentation

When looking for documentation for a specific function, you may be tempted
to run

  git -C Documentation grep index_name_pos

only to find the file technical/api-in-core-index.txt, which doesn't
help for understanding the given function. It would be better to not find
these functions in the documentation, such that people directly dive into
the code instead.

In the previous patches we have documented
* index_name_pos()
* remove_index_entry_at()
* add_[file_]to_index()
in cache.h

We already have documentation for:
* add_index_entry()
* read_index()

Which leaves us with a TODO for:
* cache -> the_index macros
* refresh_index()
* discard_index()
* ie_match_stat() and ie_modified(); how they are different and when to
  use which.
* write_index() that was renamed to write_locked_index
* cache_tree_invalidate_path()
* cache_tree_update()
Signed-off-by: NStefan Beller <sbeller@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/technical/api-in-core-index.txt

-in-core index API
-=================
-
-Talk about <read-cache.c> and <cache-tree.c>, things like:
-
-* cache -> the_index macros
-* read_index()
-* write_index()
-* ie_match_stat() and ie_modified(); how they are different and when to
-  use which.
-* index_name_pos()
-* remove_index_entry_at()
-* remove_file_from_index()
-* add_file_to_index()
-* add_index_entry()
-* refresh_index()
-* discard_index()
-* cache_tree_invalidate_path()
-* cache_tree_update()
-
-(JC, Linus)