Add defensive warning for multiple Feign Client beans to avoid potential resource exhaustion

[98001yash]

This change adds a defensive warning in FeignClientFactoryBean#getTarget()
to detect multiple Client beans defined within a Feign client context.

Although Feign clients are typically configured with singleton-scoped
Client beans, misconfiguration (such as defining multiple or non-singleton
Client beans) may lead to multiple underlying HTTP connection pools being
created, potentially resulting in resource exhaustion.

This update does not modify lifecycle behavior or bean creation logic.
It only adds a warning log to improve visibility and prevent accidental misuse.

Additionally, documentation has been updated to clarify that custom
Client beans should be singleton-scoped.