docs: add copyfile and linkfile elements description

The "copyfile" element is available since 2009 and
have been used in every Android manifest; the "linkfile"
element is available since 2014.
Now it's a good time to add both to the documentation

Change-Id: Ia987edf5f69a006235fbd3f33b744e9794a6d964
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>