[share_plus] document sharePositionOrigin for Macs (#953)
[share_plus] document sharePositionOrigin for Macs (#953)

Amend documentation to avoid confusion as in #864
fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins,fluttercommunity/plus_plugins