docs: Revised wording, section order for ShortcutManager API ref.

Provided more thorough description of static shortcut options,
integrated shortcut limits section with the "updating shortcuts"
example, added more links to narrative documentation.

Test: make ds-docs -j8

Bug: 64727234
Change-Id: I70c16082e3e85a31b501c8e313d93200d827c7f4