Alright, I'm actually bored enough to write this.
There are TWO Methods,
Method 1; Brush,
pros: Lower tris (Except when compared to patch at a distance), Textures align (somewhat) better
- Create a brush and press 'E' then manipulate the sides (See image for example).
NOTE: Keep it symmetrical. Also you may want to lower the grid size.
Method 2; Patch Mesh,
pros: Looks nicer,
Tip: With it selected hit 'S' then Natural (the texture should fit slightly better)
- Create a brush that covers the corner, then hit (Menu) Curve>Bevel followed with (Menu) Curve>Matrix>Invert.
NOTE: Patch meshes DO NOT block vis that means there MUST be structural brushes behind it otherwise your map will leak.

Edit; If you wanted to do and edge rather then a corner then either
A; Use Method 2 but don't invert it, or
B; Use the Clipper tool 'X' (click on 2d window to draw a point then draw another then hit enter (AKA 2-point clipping))