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)
latest
Status Job ID Name Coverage
  Tests
passed #144100
checks-py2

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checks-py3

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rust-cargo-test-py2

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rust-cargo-test-py3

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passed #144104
rust-cargo-test-py3-dirstate-tree

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passed #144105
test-py2

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passed #144112
test-py2-chg

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test-py2-pure

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test-py2-rust

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passed #144106
test-py3

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passed #144113
test-py3-chg

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passed #144108
test-py3-pure

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passed #144110
test-py3-rust

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

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  Phabricator
passed #144114
phabricator-refresh

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