Document default connection-pool-size for rest-client

[weltonrodrigo]

Default value for quarkus.rest-client.connection-pool-size is 50. This is the default value set by Resteasy itself that quarkus don't override by default: https://github.com/resteasy/resteasy/blob/0ccb17086c7dfffd0d6a7970db9b147fa7a0fb5c/resteasy-client/src/main/java/org/jboss/resteasy/client/jaxrs/internal/ResteasyClientBuilderImpl.java#L62

[gsmet]

Unlike its position in the tree could indicate, I think this config is used for both the classic and the reactive REST Clients. So we would need to make sure this change is correct for both of them.