Docs: add info about textdomains to the package documentation

[kbrown9]

Changes proposed in this Pull Request:

• Add info about textdomains to the package documentation.

Jetpack product discussion

We've been investigating providing a way for consumer plugins to change the textdomains in the packages. I found a couple of PRs from last year: #12525 and #12471. From the conversations in those PRs, in looks like the final decision was to let consumer plugins provide their own solution for updating textdomains in the packages.

I think this is the best approach. Consumer plugins should choose the tool and process that works best for that plugin, and there's no need for Jetpack to provide a tool for this. Instead, we should update the documentation so that consumer plugins understand that updating the textdomain is their responsibility. We can also provide links to a few tools that may be helpful, which I've done in this PR.

Does this pull request change what data or activity we track or use?

• no

Testing instructions:

• No testing is required. Just verify that the documentation update is correct, clear, and doesn't have any typos.

Proposed changelog entry for your changes:

• n/a

[jetpackbot]

Scheduled Jetpack release: January 12, 2021.
Scheduled code freeze: January 4, 2021

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Generated by 🚫 dangerJS against e947c75