InputText: clarified that callbacks cannot modify buffer when using the ReadOnly flag.

docs/CHANGELOG.txt

@@ -87,6 +87,7 @@ Other Changes:
 - InputText: numerical fields automatically accept full-width characters (U+FF01..U+FF5E)
   by converting them to half-width (U+0021..U+007E).
 - InputText: added support for shift+click style selection. (#5619) [@procedural]
+- InputText: clarified that callbacks cannot modify buffer when using the ReadOnly flag.
 - IsItemHovered: Added ImGuiHoveredFlags_DelayNormal and ImGuiHoveredFlags_DelayShort flags,
   allowing to introduce a shared delay for tooltip idioms. The delays are respectively
   io.HoverDelayNormal (default to 0.30f) and io.HoverDelayFast (default to 0.10f). (#1485)

imgui_widgets.cpp

@@ -4546,6 +4546,7 @@ bool ImGui::InputTextEx(const char* label, const char* hint, char* buf, int buf_
                     if (callback_data.SelectionEnd != utf8_selection_end || buf_dirty)      { state->Stb.select_end = (callback_data.SelectionEnd == callback_data.SelectionStart) ? state->Stb.select_start : ImTextCountCharsFromUtf8(callback_data.Buf, callback_data.Buf + callback_data.SelectionEnd); }
                     if (buf_dirty)
                     {
+                        IM_ASSERT((flags & ImGuiInputTextFlags_ReadOnly) == 0);
                         IM_ASSERT(callback_data.BufTextLen == (int)strlen(callback_data.Buf)); // You need to maintain BufTextLen if you change the text!
                         InputTextReconcileUndoStateAfterUserCallback(state, callback_data.Buf, callback_data.BufTextLen); // FIXME: Move the rest of this block inside function and rename to InputTextReconcileStateAfterUserCallback() ?
                         if (callback_data.BufTextLen > backup_current_text_length && is_resizable)