Clarify that `spring.datasource.hikari.*` properties are bound to a `HikariDataSource` instance

[wilkinsona]

The documentation lists all of the third-party properties that can be bound, but simply described them as being "Hikari specific settings". We could help users to understand what the properties are bound to by replacing this with something like "Hikari-specific settings bound to an instance of HikariDataSource".

[wilkinsona]

Closing in favor of #20727.