chore: Update setCustomAttribute jsdoc to match public docs

[dami]

Updated the setCustomAttribute comments in agent-base.js to match the latest documentation.

Overview

Updated the comments for the setCustomAttribute method to reflect the latest information from the documentation.

Related Issue(s)

Testing

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.56%. Comparing base (c573e33) to head (ce8aba9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1065 +/- ## ======================================= Coverage 87.56% 87.56% ======================================= Files 162 162 Lines 7180 7180 Branches 1406 1406 ======================================= Hits 6287 6287 Misses 778 778 Partials 115 115 ``` | [Flag](https://app.codecov.io/gh/newrelic/newrelic-browser-agent/pull/1065/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=newrelic) | Coverage Δ | | |---|---|---| | [unit-tests](https://app.codecov.io/gh/newrelic/newrelic-browser-agent/pull/1065/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=newrelic) | `76.42% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=newrelic#carryforward-flags-in-the-pull-request-comment) to find out more.

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