What

Improve the consistency of the sender/source/giver terminology in the function arguments of Tezlink operation application functions:

transfer is about the transfer operation, the account paying for the transfer is the operation's sender
transfer_tez is about moving tez from an account to another, it's at a lower-level than applying an operation and can be used in different contexts so we call the account paying for the tez movement a "giver"
burn_tez is about one account only so there is no need to specify which one.

Why

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
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