Docs: update sparse index plan with logging

The recent changes update the callers of ensure_full_index() to call
variants that will log extra information. This should assist developers
assisting users who are hitting the sparse index expansion message.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: derrickstolee

Documentation/technical/sparse-index.txt

@@ -206,3 +206,10 @@ Here are some commands that might be useful to update:
 * `git am`
 * `git clean`
 * `git stash`
+
+In order to help identify the cases where remaining index expansion is
+occurring in user machines, calls to `ensure_full_index()` have been
+replaced with `ensure_full_index_with_reason()` or with
+`ensure_full_index_unaudited()`. These versions add tracing that should
+help identify the reason for the index expansion without needing full
+access to someone's repository.