docs: warn when --ubatch-size is too small in --embeddings mode

[chinmaykamjith001]

Overview

In --embeddings mode with pooling, there was a silent HTTP 500 error due to undocumented --ubatch-size behavior. This happens when input token size exceeds --ubatch-size. This PR aims to document that behavior and include appropriate warnings to users.

In embeddings/pooling configuration, the entire input must fit within a single ubatch of --ubatch-size which is unlike normal inference where the input is split into chunks. This is not documented anywhere, leading users to set small values (e.g. 8 or 512) and get unexplained 500 errors

Changes:

1. common/arg.cpp: server-specific note to --ubatch-size --help to explain this limitation of embeddings mode
2. tools/server/server.cpp: Display startup warning when --embeddings mode is active with pooling enabled. Warns users to size --ubatch-size appropriately for their use case.
3. tools/server/README.md: Added [!IMPORTANT] callouts in both /embeddings and /reranking endpoint docs.

Additional information

Fixes #25293
Related: #6263 (fixed the crash but left the behavior undocumented)

Requirements

• I have read and agree with the contributing guidelines
  -YES

• AI usage disclosure: Yes, AI was used in final verification process of whether my code contained any apparent errosr/unintended effects.