Added missing cell execution docs for new schema

docs/format_description.rst

@@ -408,6 +408,19 @@ source_hidden   bool            Whether the cell's source should be shown
 outputs_hidden  bool            Whether the cell's outputs should be shown
 =============== =============== ==============
 
+The following metadata keys are defined at the cell level within the `execution` namespace.
+These are lower level fields capturing common kernel message timestamps for better visibility
+in applications where needed. Most users will not look at these directly.
+
+==================== ================ ==============
+Key                  Value            Interpretation
+==================== ================ ==============
+iopub.execute_input  ISO 8601 format  Indicates the time at which the kernel broadcast an execute_input message. This represents the time when request for work was received by the kernel.
+iopub.status.busy    ISO 8601 format  Indicates the time at which the iopub channel's kernel status message is 'busy'. This represents the time when work was started by the kernel.
+shell.execute_reply  ISO 8601 format  Indicates the time at which the shell channel's execute_reply status message was created. This represents the time when work was completed by the kernel.
+iopub.status.idle    ISO 8601 format  Indicates the time at which the iopub channel's kernel status message is 'idle'. This represents the time when the kernel is ready to accept new work.
+==================== ================ ==============
+
 Output metadata
 ---------------