manual: update label-pos for relative lengths

Author: Jollywatt

docs/manual.pdf

@@ -730,7 +730,7 @@ A label as an edge vertex is interpreted as the position of the node with that l
 
 
 
-== Edge types
+== Edge types <edge-types>
 
 There are three types of edges: `"line"`, `"arc"`, and `"poly"`.
 All edges have at least two `vertices`, but `"poly"` edges can have more.

docs/manual.typ

@@ -428,8 +428,12 @@
 ///    - roughly above the connector, in the case of straight lines; or
 ///    - on the outside of the curve, in the case of arcs.
 ///
-/// - label-pos (number): Position of the label along the connector, from the
-///   start to end (from `0` to `1`).
+/// - label-pos (float, ratio, relative length): Position of the label along the
+///   edge, from the start to end.
+///
+///   A number or ratio between zero and one is interpreted as a fraction of the
+///   edge length. Physical and relative relative lengths work too. For example,
+///   `100% - 1em` means `1em` from the end.
 ///
 ///   #stack(
 ///   	dir: ltr,
@@ -440,6 +444,10 @@
 ///   	),
 ///   )
 ///
+///   For `"poly"` edges (see @edge-types), a number does not specify a fraction
+///   of the path length; instead, the $k$th vertex is at position $k/n$ where
+///   $n$ is the number of vertices. Each midpoint is then at $k/n + 0.5$.
+///
 /// - label-sep (length): Separation between the connector and the label anchor.
 ///
 ///   With the default anchor (automatically set to `"south"` in this case):