Add info about shared page notification & author (Bug #1311454)

Since the owner of a page who shares it with the
group does not receive notifications anymore as
per Bug #1267861, the lang string for the help
text needed updating.

Change-Id: Ie5abcdae9b839fb4919f14f1fa5c6b33dfd271c7
Signed-off-by: Kristina D.C. Hoeppner <kristina@catalyst.net.nz>