Add documentation for caching in a module (#14026)

* Add documentation for caching in a module

* Changelog

* Formatting

* Wrap lines at a length that mdbook is happier with

* Typo fix

Co-authored-by: Erik Johnston <erik@matrix.org>

* Link to recent version of the API

In the longer term I'd like to see us generate markdown with Sphinx.

* Refer to public `cached` decorator

* Mark caching as being added in 1.74

Some of the underlying infrastructure was added in 1.69, but the
public-facing `cached` decorator was only added in 1.74. It is the
latter that I think we should be advertising.

* Update docs/modules/writing_a_module.md

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

---------

Co-authored-by: David Robertson <davidr@element.io>
Co-authored-by: Erik Johnston <erik@matrix.org>
Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

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