Fix some typos and grammaros hphp/doc/php.extension.compat.layer.

Summary: The content still smells as an "as planned" document, rather than as
an "as built" document.
Closes https://github.com/facebook/hhvm/pull/4346

Reviewed By: @​svcscm

Differential Revision: D1707235

Pulled By: @fredemmott