Improve commentary of prepare_to_modify_buffer_1

* src/insdel.c (prepare_to_modify_buffer_1): Mention in commentary
that this function runs Lisp.  Suggested by Richard Stallman
<rms@gnu.org>.

1 files changed, 6 insertions, 1 deletions

diff --git a/src/insdel.c b/src/insdel.c
index 8b08c1e59b7..bb3171b14ce 100644
--- a/src/insdel.c
+++ b/src/insdel.c
@@ -1785,7 +1785,12 @@ run_undoable_change (void)
    any modification properties the text may have.
 
    If PRESERVE_PTR is nonzero, we relocate *PRESERVE_PTR
-   by holding its value temporarily in a marker.  */
+   by holding its value temporarily in a marker.
+
+   This function runs Lisp, which means it can GC, which means it can
+   compact buffers, including the current buffer being worked on here.
+   So don't you dare calling this function while manipulating the gap,
+   or during some other similar "critical section".  */
 
 void
 prepare_to_modify_buffer_1 (ptrdiff_t start, ptrdiff_t end,