RelNotes/2.50.0: fix typos & other improvements

RelNotes/2.50.0: fix typos & other improvements

• Replace with phrases that are more standard (“all-or-nothing”
  instead of “-none”)
• Add coordinating words that make it less likely for you to trip
  over the sentence (“*that* "gc" can do”)
• Use “SMTP” instead of both SMTP and smtp
• Don’t mention `git fsck --reference` since the previous release
  was not affected by this minor bug.  Also say “errored out” since
  the git-refs(1) bug was there in v2.48.0 as well
• Use the more widespread “linked” instead of “secondary worktree”

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>