Clarify place documentation (#5031)

crates/typst/src/layout/place.rs

@@ -1,26 +1,70 @@
 use crate::foundations::{elem, scope, Cast, Content, Smart};
 use crate::layout::{Alignment, Em, Length, Rel};
 
-/// Places content at an absolute position.
+/// Places content relatively to its parent container.
 ///
-/// Placed content will not affect the position of other content. Place is
-/// always relative to its parent container and will be in the foreground of all
-/// other content in the container. Page margins will be respected.
+/// Placed content can be either overlaid (the default) or floating. Overlaid
+/// content is aligned with the parent container according to the given
+/// [`alignment`]($place.alignment), and shown over any other content added so
+/// far in the container. Floating content is placed at the top or bottom of
+/// the container, displacing other content down or up respectively. In both
+/// cases, the content position can be adjusted with [`dx`]($place.dx) and
+/// [`dy`]($place.dy) offsets without affecting the layout.
 ///
+/// The parent can be any container such as a [`block`], [`box`],
+/// [`rect`], etc. A top level `place` call will place content directly
+/// in the text area of the current page. This can be used for absolute
+/// positioning on the page: with a `top + left`
+/// [`alignment`]($place.alignment), the offsets `dx` and `dy` will set the
+/// position of the element's top left corner relatively to the top left corner
+/// of the text area. For absolute positioning on the full page including
+/// margins, you can use `place` in [`page.foreground`]($page.foreground) or
+/// [`page.background`]($page.background).
 ///
-/// # Example
+/// # Examples
 /// ```example
 /// #set page(height: 60pt)
 /// Hello, world!
 ///
+/// #rect(
+///   width: 100%,
+///   height: 2cm,
+///   place(horizon + right, square()),
+/// )
+///
 /// #place(
-///   top + right,
-///   square(
-///     width: 20pt,
-///     stroke: 2pt + blue
-///   ),
+///   top + left,
+///   dx: -5pt,
+///   square(size: 5pt, fill: red),
 /// )
 /// ```
+///
+/// # Effect on the position of other elements
+///
+/// Overlaid elements don't take space in the flow of content, but a `place`
+/// call inserts an invisible block-level element in the flow. This can
+/// affect the layout by breaking the current paragraph. To avoid this,
+/// you can wrap the `place` call in a [`box`] when the call is made
+/// in the middle of a paragraph. The alignment and offsets will then be
+/// relative to this zero-size box. To make sure it doesn't interfere with
+/// spacing, the box should be attached to a word using a zero-width joiner.
+///
+/// For example, the following defines a function for attaching an annotation
+/// to the following word:
+///
+/// ```example
+/// #let annotate(..args) = {
+///   box(place(..args))
+///   sym.zwj
+///   h(0pt, weak: true)
+/// }
+///
+/// A placed #annotate(square(), dy: 2pt)
+/// square in my text.
+/// ```
+///
+/// The zero-width weak spacing serves to discard spaces between the function
+/// call and the next word in markup.
 #[elem(scope)]
 pub struct PlaceElem {
     /// Relative to which position in the parent container to place the content.
@@ -28,9 +72,8 @@ pub struct PlaceElem {
     /// - If `float` is `{false}`, then this can be any alignment other than `{auto}`.
     /// - If `float` is `{true}`, then this must be `{auto}`, `{top}`, or `{bottom}`.
     ///
-    /// When an axis of the page is `{auto}` sized, all alignments relative to
-    /// that axis will be ignored, instead, the item will be placed in the
-    /// origin of the axis.
+    /// When `float` is `{false}` and no vertical alignment is specified, the
+    /// content is placed at the current position on the vertical axis.
     #[positional]
     #[default(Smart::Custom(Alignment::START))]
     pub alignment: Smart<Alignment>,
@@ -73,7 +116,8 @@ pub struct PlaceElem {
     /// ```
     pub float: bool,
 
-    /// The amount of clearance the placed element has in a floating layout.
+    /// The spacing between the placed element and other elements in a floating
+    /// layout.
     ///
     /// Has no effect if `float` is `{false}`.
     #[default(Em::new(1.5).into())]