Blender Git Loki

UI: Improve UV sticky select mode description

The sticky_select_mode property is an enum where each item has a
different behavior, but currently the description is for a single one
of the items. It should be more general, especially because one of the
items is "Disabled."

A couple more problems:
- "Automatically" is not so helpful. Many things any program does are
"automatic", it doesn't really help to say it.
- "Also" is bad grammar.
- Overly complex wording

This also adjusts the SHARED_VERTEX tooltip, because "irrespective"
should have "of" after, but it's also unecessarily complex.

Differential Revision: https://developer.blender.org/D9049