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

source/blender/makesrna/intern/rna_space.c

@@ -2836,7 +2836,7 @@ static void rna_def_space_image_uv(BlenderRNA *brna)
        "SHARED_VERTEX",
        ICON_STICKY_UVS_VERT,
        "Shared Vertex",
-       "Select UVs that share mesh vertex, irrespective if they are in the same location"},
+       "Select UVs that share a mesh vertex, whether or not they are at the same location"},
       {0, NULL, 0, NULL, NULL},
   };
 
@@ -2864,9 +2864,7 @@ static void rna_def_space_image_uv(BlenderRNA *brna)
   RNA_def_property_enum_sdna(prop, NULL, "sticky");
   RNA_def_property_enum_items(prop, sticky_mode_items);
   RNA_def_property_ui_text(
-      prop,
-      "Sticky Selection Mode",
-      "Automatically select also UVs sharing the same vertex as the ones being selected");
+      prop, "Sticky Selection Mode", "Method for extending UV vertex selection");
   RNA_def_property_update(prop, NC_SPACE | ND_SPACE_IMAGE, NULL);
 
   /* drawing */