Document that `doSave()` can be called before `start()` for IXMLExtension

[datho7561]

Clarify that the doSave() method can be called before the start() method in the documentation for IXMLExtension. For instance, this is done in the XMLAssert tests to test LemMinX extensions.

[angelozerr]

I wonder more and more if we must remove this doSave and have a participant which tracks didOpen, didSave, etc and have a participant which tracks settings (see WorkspaceService PR did by @mickaelistria https://github.com/eclipse/lemminx/pull/958)

@rzgry I think you could be interested too by this issue since you are using doSave too.

[rzgry]

We currently use doSave just for updating the settings and keeping them updated. https://github.com/OpenLiberty/liberty-language-server/blob/master/lemminx-liberty/src/main/java/io/openliberty/lemminx/liberty/LibertyExtension.java#L52

A setting participant that keeps track of settings and is notified whenever the settings are changed would be useful.