debugsharesafe: recommend from `debugupgraderepo` and `help -e share -v`

Recommending one debug command from another one sounds fine to me.

Also documentation about share-safe in `hg help -e share -v` was lacking correct
description on how upgrade existing shares. This fixes that and also start
mentioning about debugsharesafe.

Differential Revision: https://phab.mercurial-scm.org/D9571
15 jobs for topic/default/share-safe in 27 minutes and 51 seconds (queued for 4 seconds)
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  Tests
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passed test-py2-pure #144107

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passed test-py3 #144106

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passed test-py3-pure #144108

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passed test-py3-rust-dirstate-tree #144111

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  Phabricator
passed phabricator-refresh #144114

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