Update docs for FragmentData

We create multiple FragmentData objects for all fragmented layout
objects, not just self-painting layers.

Change-Id: Ib95bda3415b3bd7570f74f26746adcea8c35eafe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913096Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715116}

third_party/blink/renderer/core/paint/README.md

@@ -579,15 +579,15 @@ IsolationPiercing vs IsolationBlocked subtree update reasons.
 #### Fragments
 
 In the absence of multicolumn/pagination, there is a 1:1 correspondence between
-self-painting `PaintLayer`s and `FragmentData`. If there is
-multicolumn/pagination, there may be more `FragmentData`s.. If a `PaintLayer`
-has a property node, each of its fragments will have one. The parent of a
-fragment's property node is the property node that belongs to the ancestor
-`PaintLayer` which is part of the same column. For example, if there are 3
-columns and both a parent and child `PaintLayer` have a transform, there will be
-3 `FragmentData` objects for the parent, 3 for the child, each `FragmentData`
-will have its own `TransformPaintPropertyNode`, and the child's ith fragment's
-transform will point to the ith parent's transform.
+`LayoutObject`s and `FragmentData`. If there is multicolumn/pagination,
+there may be more `FragmentData`s. If a `LayoutObject` has a property node,
+each of its fragments will have one. The parent of a fragment's property node is
+the property node that belongs to the ancestor `LayoutObject` which is part of
+the same column. For example, if there are 3 columns and both a parent and child
+`LayoutObject` have a transform, there will be 3 `FragmentData` objects for
+the parent, 3 for the child, each `FragmentData` will have its own
+`TransformPaintPropertyNode`, and the child's ith fragment's transform will
+point to the ith parent's transform.
 
 Each `FragmentData` receives its own `ClipPaintPropertyNode`. They
 also store a unique `PaintOffset, `PaginationOffset and

third_party/blink/renderer/core/paint/fragment_data.h

@@ -16,8 +16,6 @@ namespace blink {
 class PaintLayer;
 
 // Represents the data for a particular fragment of a LayoutObject.
-// Only LayoutObjects with a self-painting PaintLayer may have more than one
-// FragmentData, and even then only when they are inside of multicol.
 // See README.md.
 class CORE_EXPORT FragmentData {
   USING_FAST_MALLOC(FragmentData);