man-db: caveat: gapropos/gwhatis require gmandb

[krobelus]

As reported in https://github.com/fish-shell/fish-shell/discussions/12062,

$ brew install man-db
$ echo >>~/.zprofile 'PATH="/opt/homebrew/opt/man-db/libexec/bin:$PATH"'

breaks commands like

$ apropos zsh
zsh: nothing appropriate

until the user runs "gmandb".

This has caused confusion because fish shell uses "apropos" to compute completions for the "man" command.

Maybe we can find a reliable way to:

1. either run "gmandb" automatically as needed (maybe we should ask "man-db" upstream to make this the responsibility of "apropos" and "whatis").
2. or at least detect this case (user trying to use "gaprops" with empty "/opt/homebrew/var/cache/man/") and tell them to run "gmandb".

Until then, add a caveat telling the user to run "gmandb" (or "mandb" I guess).

At the risk of being patronizing, emphasize that this is more likely to be necessary if the user makes "apropos" run "gapropos" because of cases like fish. Happy to remove this.

[github-actions[bot]]

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[krobelus]

yikes