Added some doc comments

src/Dimensions.zig

@@ -29,10 +29,16 @@ pub const Dimension = enum {
     }
 };
 
+// --------- Dimensions struct ---------
+
+/// Holds the exponent of each SI base dimension for a given quantity (e.g. velocity = L¹T⁻¹).
+/// All values are `comptime_int` — no runtime storage.
 const Self = @This();
 
 data: std.EnumArray(Dimension, comptime_int),
 
+/// Create a `Dimensions` from an anonymous struct literal, e.g. `.{ .L = 1, .T = -1 }`.
+/// Unspecified dimensions default to 0.
 pub fn init(comptime init_val: anytype) Self {
     var s = Self{ .data = std.EnumArray(Dimension, comptime_int).initFill(0) };
     inline for (std.meta.fields(@TypeOf(init_val))) |f|
@@ -52,6 +58,7 @@ pub fn set(comptime self: *Self, comptime key: Dimension, comptime val: i8) void
     self.data.set(key, val);
 }
 
+/// Add exponents component-wise. Used internally by `mulBy`.
 pub fn add(comptime a: Self, comptime b: Self) Self {
     var result = Self.initFill(0);
     for (std.enums.values(Dimension)) |d|
@@ -59,6 +66,7 @@ pub fn add(comptime a: Self, comptime b: Self) Self {
     return result;
 }
 
+/// Subtract exponents component-wise. Used internally by `divBy`.
 pub fn sub(comptime a: Self, comptime b: Self) Self {
     @setEvalBranchQuota(10_000);
     var result = Self.initFill(0);
@@ -67,6 +75,7 @@ pub fn sub(comptime a: Self, comptime b: Self) Self {
     return result;
 }
 
+/// Returns true if every dimension exponent is equal. Used to enforce type compatibility in `add`, `sub`, `to`.
 pub fn eql(comptime a: Self, comptime b: Self) bool {
     inline for (std.enums.values(Dimension)) |d|
         if (a.get(d) != b.get(d)) return false;