Document asdf plugin list all/asdf plugin add <name> perform plugin repo sync based on asdfrc plugin_repository_last_check_duration option

[jthegedus]

Is your feature request related to a problem? Please describe.

People have questioned why the plugin they added (to the plugin repo) is not installable via the short-name from the plugin repo moments after the plugin has been added to the repo.

Usually, the user is aware that the plugin repo syncs, however it's not clear when (I believe once daily) and most aren't aware they can force the sync via the asdf plugin list all command.

asdf plugin add <name> and asdf plugin list all will perform a check at most once per hour (since #957). This value is also configurable via .asdfrc since #957

Describe the solution you'd like

Document this in the docs site page: https://asdf-vm.com/#/core-manage-plugins?id=update

It is currently documented in the config section https://asdf-vm.com/#/core-configuration?id=homeasdfrc but is asked often enough we should mention it elsewhere.

Describe similar asdf features and why they are not sufficient

Describe workarounds you've considered

Additional context

Any question we are asked via an issue should be added to the docs site or the functionality in question have its first point of user contact improved.