[improve][doc] Clarify producer name uniqueness and Key_Shared batching requirements

[kevin-pan-skydio]

Motivation

While working with Pulsar, I noticed a couple of areas where the docs could be
a bit more explicit to help new users avoid common pitfalls:

1. Producer name uniqueness — The existing docs mention producer names but
   don't prominently call out that explicitly assigned names must be unique per topic.
   Adding this detail up front can save users some head-scratching.

2. Key_Shared batching requirement — The docs do cover the need for
   key-based batching with Key_Shared subscriptions, but this information
   lives fairly deep in the section. A brief note near the top of the
   Key_Shared description could help users discover this requirement sooner.

Modifications

• docs/concepts-messaging.md — Expanded the producer name description in
  the message components table. Added a note near the top of the Key_Shared
  section linking to the existing batching details below.
• docs/concepts-clients.md — Added a short "Producer naming" subsection
  covering uniqueness, auto-generation, and when explicit names are helpful.
• docs/client-libraries-producers.md — Added a "Producer naming"
  subsection with code examples (Java, C++, Python) and a note that meaningful
  names help with debugging.
• docs/client-libraries-consumers.md — Added a brief note at the top of
  the Key_shared consumer section pointing out the producer-side batching
  requirement.

Documentation

• doc-required

Matching

No code changes. Documentation only.

[lhotari]

LGTM, great work @kevin-pan-skydio