Replaced Markdown syntax with HTML syntax for hyperlinks in documentation comments. HTML links work both inside Xcode (whether Objective-C or Swift) and on the Web (via jazzy).
Fixes#3961.
Cherry-picked from de0705c7ed.
Include offline headers in the umbrella header used by the static framework. These includes are already present in the umbrella header used by the dynamic framework. jazzy relies on the static umbrella header to include all the headers, so the offline headers were also missing from documentation.
