Documentation: tweak use case in "git stash save --keep-index"

The documentation suggests using "git stash apply" in the
--keep-index workflow even though doing so will lead to clutter
in the stash.  And given that the changes are about to be
committed anyway "git stash pop" is more sensible.

Additionally the text preceeding the example claims that it
works for "two or more commits", but the example itself is
really tailored for just two.  Expanding it just a little
makes it clear how the procedure generalizes to N commits.

Finally the example is annotated with some commentary to
explain things on a line-by-line basis.

Documentation/git-stash.txt

@@ -180,13 +180,14 @@ each change before committing:
 +
 ----------------------------------------------------------------
 ... hack hack hack ...
-$ git add --patch foo
-$ git stash save --keep-index
-$ build && run tests
-$ git commit -m 'First part'
-$ git stash apply
-$ build && run tests
-$ git commit -a -m 'Second part'
+$ git add --patch foo            # add just first part to the index
+$ git stash save --keep-index    # save all other changes to the stash
+$ edit/build/test first part
+$ git commit foo -m 'First part' # commit fully tested change
+$ git stash pop                  # prepare to work on all other changes
+... repeat above five steps until one commit remains ...
+$ edit/build/test remaining parts
+$ git commit foo -m 'Remaining parts'
 ----------------------------------------------------------------
 
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