Fix remaining provider config settings to be decisive in value

Fix remaining provider config settings to be decisive in value

There is one remaining config setting for providers, soft_load, which is
enabled when provided in a config, regardless of its value.  Augment it
to require a decisive value 1/0, yes/no, on/off, true/false, as we've
recently done for the activate setting.

Also, since it wasn't previously documented, add docs for it.

Fixes #23105

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23111)