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.

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