refactor: Small clarifications to authz docs and one error description

[ivan-gavran]

Description

This small PR is trying to clarify the documentation that I found ambiguous when reading it.
Furthermore, it makes one error message more precise ("non-negative" --> "positive").

In particular:

• I clarified that the field AcceptResponse.Accept will be set to true when an authorization is accepted, but will not be set to false otherwise (instead, the function Accept will return an error).
• I clarified that the field AcceptResponse.Updated will not always be populated: it will be nil unless there are real changes to the authorization
• I changed the error message in the send_authorization.go function, when IsAllPositive() returns false from spend limit cannot be negitive to spend limit must be positive. I emphasized the fact that spend limit must be positive in the documentation.

This is my first contribution to the Cosmos-SDK codebase so if I made some mistake the process (wrt the checklist), guide me patiently, pls.

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Author Checklist

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I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification: NOTE: does not apply, these are clarification changes
• followed the guidelines for building modules: NOTE: does not apply, these are clarification changes
• included the necessary unit and integration tests: NOTE: does not apply, these are clarification changes
• added a changelog entry to CHANGELOG.md: NOTE: does not apply, these are clarification changes
• included comments for documenting Go code NOTE: does not apply, these are clarification changes
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

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I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[atheeshp]

lgtm, thanks @ivan-gavran !