Add docs for strategies.yaml hash_string

[rob05c]

Documents an undocumented field. Because I got tired of looking it up in the code.

This applies to both core and parent_select plugin strategies.

Should be backported to 9.2, 9.1, and 9.0. The config has existed since 9.0, and there's no reason not to document it there.

Also clarified that host is the FQDN. Prior docs could have been interpreted as the short hostname.

[traeak]

[approve ci]

[ezelkow1]

[approve ci]

[rob05c]

[approve ci docs]

[bryancall]

[approve ci autest]

[zwoop]

Cherry-picked to v9.2.x