[Buttons] Updated docs to match actual API.
[Buttons] Updated docs to match actual API.

Summary:
Both the Objective-C and Swift code samples in the docs were out of date, adjusted them to match the actual API. Also, some minor formatting changes for the intro text.

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Closes https://github.com/google/material-components-ios/pull/129
GitHub author: peterfriese

!![[https://github.com/google/material-components-ios/pull/129 | This is a child of GitHub pull request #129]]!!.

Reviewers: #mdc_ios_owners, github-bot

Reviewed By: #mdc_ios_owners

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D187

randallli/material-components-ios,chriscox/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,brianjmoore/material-components-ios,brianjmoore/material-components-ios,samnm/material-components-ios,willlarche/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,sigito/material-components-ios,Eccelor/material-components-ios,Eccelor/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,samnm/material-components-ios,sigito/material-components-ios,Eccelor/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,Eccelor/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,samnm/material-components-ios,Eccelor/material-components-ios,randallli/material-components-ios,sigito/material-components-ios,willlarche/material-components-ios,sigito/material-components-ios,material-components/material-components-ios,chriscox/material-components-ios,brianjmoore/material-components-ios,chriscox/material-components-ios