doc(emath): Add top_left as an alias for left_top, etc. (emilk#4689)

# What

Adds `#[doc(alias = "top_left")]` as an alias for `left_top`, and so on
for `right_top`, `right_bottom`, `left_bottom`.

# Why

Extremely minor doc-only change, but I keep going to type "top left" to
look for the top left of a rectangle.
I'm unsure whether this is just a british-english thing or an
english-in-general thing, but `top left corner` is far more common than
`left top corner`.
These doc aliases don't conflict with anything, and mean that
rust-analyzer will suggest the correct function when I search for the
wrong thing.

This improves ergonomics and discoverability in my opinion, even if not
by much.

crates/emath/src/rect.rs

@@ -565,6 +565,7 @@ impl Rect {
     }
 
     #[inline(always)]
+    #[doc(alias = "top_left")]
     pub fn left_top(&self) -> Pos2 {
         pos2(self.left(), self.top())
     }
@@ -575,6 +576,7 @@ impl Rect {
     }
 
     #[inline(always)]
+    #[doc(alias = "top_right")]
     pub fn right_top(&self) -> Pos2 {
         pos2(self.right(), self.top())
     }
@@ -590,6 +592,7 @@ impl Rect {
     }
 
     #[inline(always)]
+    #[doc(alias = "bottom_left")]
     pub fn left_bottom(&self) -> Pos2 {
         pos2(self.left(), self.bottom())
     }
@@ -600,6 +603,7 @@ impl Rect {
     }
 
     #[inline(always)]
+    #[doc(alias = "bottom_right")]
     pub fn right_bottom(&self) -> Pos2 {
         pos2(self.right(), self.bottom())
     }