docs(material/button-toggle): add description for appearance property in default options (#22852)
docs(material/button-toggle): add description for appearance property in default options (#22852)

* docs(material/button-toggle): add description for appearance property in default options

Developers can specify a default appearance for button toggles
using the `MatButtonToggleDefaultOptions`. The appearance property
within these default options is not documented. This commit adds
a description.

* fixup! docs(material/button-toggle): add description for appearance property in default options

Address feedback
angular/components,angular/components,angular/components,crisbeto/material2,josephperrott/material2,crisbeto/material2,mmalerba/material2,josephperrott/material2,mmalerba/material2,DevVersion/material2,crisbeto/material2,josephperrott/material2,DevVersion/material2,DevVersion/material2,DevVersion/material2,josephperrott/material2,mmalerba/material2,crisbeto/material2,angular/components,josephperrott/material2,mmalerba/material2,DevVersion/material2,mmalerba/material2