Problem
Throughout the codebase, when the safeTransferFrom function is used to transfer assets into the
system from an external address there is an implicit requirement that the external address has already
granted the appropriate approvals.
For instance:

• The proposeRootBundle function relies on safeTransferFrom which requires that HubPool has
  been granted an allowance of bondAmount bondTokens by the caller.
• The addLiquidity function relies on safeTransferFrom, requiring that the HubPool has been
  granted an l1TokenAmount allowance of the caller's l1Token.

Solution:
This PR addresses this issue by adding additional comments.