docs(proto): specify time type in vesting proto (#14513)

## Description

Updates documentation to specify time type for vesting accounts.

Context: I've been creating new vesting accounts and had to go back reading code make sure about the exact type of the end / start time.

---

### 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...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] 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)

proto/cosmos/vesting/v1beta1/tx.proto

@@ -45,6 +45,7 @@ message MsgCreateVestingAccount {
     (gogoproto.castrepeated) = "github.com/cosmos/cosmos-sdk/types.Coins"
   ];
 
+  // end of vesting as unix time
   int64 end_time = 4;
   bool  delayed  = 5;
 }
@@ -85,8 +86,9 @@ message MsgCreatePeriodicVestingAccount {
 
   option (gogoproto.equal) = false;
 
-  string          from_address    = 1;
-  string          to_address      = 2;
+  string from_address = 1;
+  string to_address   = 2;
+  // start of vesting as unix time
   int64           start_time      = 3;
   repeated Period vesting_periods = 4 [(gogoproto.nullable) = false, (amino.dont_omitempty) = true];
 }