BUG #15912: The units of `autovacuum_vacuum_cost_delay` setting should be documented

The following bug has been logged on the website:

Bug reference:      15912
Logged by:          Basil Bourque
Email address:      basil.bourque@gmail.com
PostgreSQL version: 12beta2
Operating system:   macOS
Description:

The `autovacuum_vacuum_cost_delay` setting:

https://www.postgresql.org/docs/devel/runtime-config-autovacuum.html#GUC-AUTOVACUUM-VACUUM-COST-DELAY

…does not specify its units (seconds? milliseconds?) explicitly.  

Previously, one could guess that the units were milliseconds. 

But now in Postgres 12 the argument type changed from `integer` to `floating
point`. So now the units are quite unclear. I am guessing a whole number
means milliseconds, while the fractional part represents microseconds. But
that is only a guess. Should be documented.