[docs] document eval events and callback references

[vishruthb]

What does this PR do?

Documents the standalone eval event flow for direct Trainer.eval() calls in the Event API docs.

This also fixes stale event references in event/callback documentation:

• BEFORE_EVAL_ALL -> EVAL_BEFORE_ALL
• AFTER_EVAL_ALL -> EVAL_AFTER_ALL
• EVAL_BATCH_FORWARD -> EVAL_BEFORE_FORWARD
• PREDICT_BATCH_FORWARD -> PREDICT_BEFORE_FORWARD

It also clarifies that EVAL_END fires once per evaluator/validation dataset.

What issue(s) does this change relate to?

• Fixes EVAL_STANDALONE_END documentation missing #3443

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[vishruthb]

hi @bowenyang008, wanted to quickly bump this when you have a chance. happy to make changes as you see fit!