Update staking.adoc

[saniksin]

Description of the Changes

Clarified the difference between enter_delegation_pool and add_to_delegation_pool functions in the staking documentation.

• Specified that enter_delegation_pool should be used when a user delegates for the first time to a pool.
• Explained that add_to_delegation_pool is intended for adding more tokens after the initial delegation.

These changes aim to eliminate confusion between the two similar-sounding functions and help users avoid mistakes when interacting with the delegation contract.

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Check List

• Changes made against main branch and PR does not conflict
• PR title is meaningful, e.g: clarify usage of delegation pool entry functions
• Detailed description added under "Description of the Changes"
• Specific URL(s) added under "PR Preview URL"

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