doc(stash): clarify the description of `save`

doc(stash): clarify the description of `save`

The original phrasing of this paragraph made at least one person stumble
over the word "from" (thinking that it was a typo and "from" was
intended), and other readers chimed in, agreeing that it was confusing:
https://public-inbox.org/git/0102016b8d597569-c1f6cfdc-cb45-4428-8737-cb1bc30655d8-000000@eu-west-1.amazonses.com/#t

Let's rewrite that paragraph for clarity.

Inspired-by-a-patch-by: Catalin Criste <cris_linu_w@yahoo.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>