## 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.
---
### Author Checklist
*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*
I have...
- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [x] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#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](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules): NOTE: does not apply, these are clarification changes
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing): 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](https://blog.golang.org/godoc) NOTE: does not apply, these are clarification changes
- [x] updated the relevant documentation or specification
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed
### Reviewers Checklist
*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*
I have...
- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) 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)
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