scratch/undo-apply ae89b15a0ad 2/2: Document the new undo-list entry kind

branch: scratch/undo-apply
commit ae89b15a0ad16ed426bec4928eb2c6722d43f015
Author: Helmut Eller <[email protected]>
Commit: Helmut Eller <[email protected]>

    Document the new undo-list entry kind
    
    * src/buffer.c (buffer-undo-list): Update the docstring.
    * doc/lispref/text.texi (buffer-undo-list): Describe the new entry kind.
---
 doc/lispref/text.texi | 7 +++++++
 src/buffer.c          | 6 ++++++
 2 files changed, 13 insertions(+)

diff --git a/doc/lispref/text.texi b/doc/lispref/text.texi
index db2410fbf30..68f9a80b5d3 100644
--- a/doc/lispref/text.texi
+++ b/doc/lispref/text.texi
@@ -1449,6 +1449,13 @@ arguments @var{args}.
 This kind of element enables undo limited to a region to determine
 whether the element pertains to that region.
 
+@item (apply @var{delta} (@var{beg} . @var{end}) @var{funname} . @var{args})
+This kind of element is similar to the previous form.  The difference is
+that @var{funname} is invoked with @w{@code{(apply @var{funname}
+@var{beg2} @var{end2} @var{args})}}, where @var{beg2} and @var{end2} are
+the adjusted values of @var{beg} and @var{end}.  The function can use
+those arguments to compute positions relative to the adjusted region.
+
 @item nil
 This element is a boundary.  The elements between two boundaries are
 called a @dfn{change group}; normally, each change group corresponds to
diff --git a/src/buffer.c b/src/buffer.c
index 0a2bff27c11..e302117b1bc 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5761,6 +5761,12 @@ in the active region.  BEG and END is the range affected by this entry
 and DELTA is the number of characters added or deleted in that range by
 this change.
 
+An entry (apply DELTA (BEG . END) FUN-NAME . ARGS) is similar to the
+previous kind.  The difference is that Emacs invokes FUN-NAME with
+\(apply FUN-NAME BEG2 END2 ARGS), where BEG2 and END2 are the adjusted
+values of BEG and END.  The function can use those arguments to compute
+positions relative to the adjusted region.
+
 An entry (MARKER . DISTANCE) indicates that the marker MARKER
 was adjusted in position by the offset DISTANCE (an integer).