[chore] Go back to using `name` for attachment alt text for now, add note in docs (#4559)v0.20.2

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

This pull request switches us back to using the `name` property instead of the `summary` property for media descriptions for now, as using `summary` was causing accessibility issues with folks on *key, *oma, and Iceshrimp instances not being able to see GtS alt text, as those softwares (as of their latest releases I think) only look for `name`.

Also updates the docs to add a note there on our intention to switch to using the HTML `summary` property instead of / in addition to `name` from v0.21.0 onwards.

closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4524

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4559
Reviewed-by: Daenney <daenney@noreply.codeberg.org>
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

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