Document why Basis columns are a, b, and c

Why Basis doesn't follow Godot's column name convention came up in
Discord, and the reasoning was fairly buried in code reviews and commits
in other projects, so I'm sharing the knowledge with API users.

The explanation here is lifted directly from
godot-rust/gdnative@16aa18e

Author: fpdotmonkey

godot-core/src/builtin/basis.rs

@@ -23,6 +23,12 @@ use std::ops::{Mul, MulAssign};
 ///
 /// The basis vectors are the columns of the matrix, whereas the [`rows`](Self::rows) field represents
 /// the row vectors.
+///
+/// Note that the names of the column vectors here are `a`, `b`, and `c`, which differs from Godot's convention of `x`, `y`, and `z`.  This is
+/// because columns are the basis vectors of the transform, while rows represent the X/Y/Z coordinates of each vector. Although basis vectors are
+/// the _transformed_ unit vectors of X/Y/Z axes, they have no direct relation to those axes in the _transformed_ coordinate system. Thus, an
+/// independent notion of a, b, c does not suggest such a relation.  Furthermore, there are sometimes expressions such as `x.x`, `x.y`, `y.x`
+/// etc. They are typically hard to read and error-prone to write. Having `a.x`, `a.y`, `b.x` makes things more understandable.
 #[derive(Copy, Clone, PartialEq, Debug)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 #[repr(C)]
@@ -457,7 +463,7 @@ impl Basis {
 
     /// Returns the first column of the matrix,
     ///
-    /// _Godot equivalent: `Basis.x`_
+    /// _Godot equivalent: `Basis.x`_, see [`Basis`] for why it's changed
     #[doc(alias = "x")]
     #[must_use]
     pub fn col_a(&self) -> Vector3 {
@@ -473,7 +479,7 @@ impl Basis {
 
     /// Returns the second column of the matrix,
     ///
-    /// _Godot equivalent: `Basis.y`_
+    /// _Godot equivalent: `Basis.y`_, see [`Basis`] for why it's changed
     #[doc(alias = "y")]
     #[must_use]
     pub fn col_b(&self) -> Vector3 {
@@ -489,7 +495,7 @@ impl Basis {
 
     /// Returns the third column of the matrix,
     ///
-    /// _Godot equivalent: `Basis.z`_
+    /// _Godot equivalent: `Basis.z`_, see [`Basis`] for why it's changed
     #[doc(alias = "z")]
     #[must_use]
     pub fn col_c(&self) -> Vector3 {